Semiconductor Device and Manufacturing Method Thereof

ABSTRACT

To provide a semiconductor device having a structure with which the device can be easily manufactured even if the size is decreased and which can suppress a decrease in electrical characteristics caused by the decrease in the size, and a manufacturing method thereof. A source electrode layer and a drain electrode layer are formed on an upper surface of an oxide semiconductor layer. A side surface of the oxide semiconductor layer and a side surface of the source electrode layer are provided on the same surface and are electrically connected to a first wiring. Further, a side surface of the oxide semiconductor layer and a side surface of the drain electrode layer are provided on the same surface and are electrically connected to a second wiring.

This application is a continuation of copending U.S. application Ser.No. 14/272,767, filed on May 8, 2016 which is incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to an object, a method, or a manufacturingmethod. The present invention relates to a process, a machine,manufacture, or a composition of matter. In particular, one embodimentof the present invention relates to a semiconductor device, a displaydevice, a light-emitting device, a storage device, an arithmetic unit,an imaging device, a method for driving any of them, or a method formanufacturing any of them.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A transistor and a semiconductor circuit areembodiments of semiconductor devices. In some cases, a storage device, adisplay device, or an electronic device includes a semiconductor device.

BACKGROUND ART

Attention has been focused on a technique for forming a transistor usinga semiconductor thin film formed over a substrate having an insulatingsurface (also referred to as a thin film transistor (TFT)). Thetransistor is used in a wide range of electronic devices such as anintegrated circuit (IC) and an image display device (display device). Asilicon-based semiconductor material is widely known as a material for asemiconductor thin film applicable to a transistor. As another example,an oxide semiconductor has attracted attention.

For example, a transistor whose active layer includes an amorphous oxidesemiconductor containing indium (In), gallium (Ga), and zinc (Zn) isdisclosed in Patent Document 1.

REFERENCE Patent Document [Patent Document 1] Japanese Published PatentApplication No. 2006-165528 DISCLOSURE OF INVENTION

A high density of an integrated circuit requires miniaturization of atransistor, and a transistor having a simple structure and a simplemanufacturing method of a transistor are required because theminiaturization increases the degree of difficulty of a manufacturingprocess.

In addition, it is known that miniaturization of a transistor is likelyto cause deterioration of or variation in electrical characteristics ofthe transistor. In other words, miniaturization of a transistor islikely to cause a decrease in yield of an integrated circuit.

Thus, one object of one embodiment of the present invention is toprovide a semiconductor device having a structure with which the devicecan be manufactured through a simple process even in the case ofminiaturization. Another object is to provide a semiconductor devicehaving a structure with which a decrease in a yield due tominiaturization can be suppressed. Another object of one embodiment ofthe present invention is to provide a semiconductor device in whichdeterioration of electrical characteristics which becomes morenoticeable as the transistor is miniaturized can be suppressed. Anotherobject is to provide a semiconductor device having a high degree ofintegration. Another object is to provide a semiconductor device inwhich deterioration of electrical characteristics is reduced. Anotherobject is to provide a semiconductor device in which variation inelectrical characteristics is reduced. Another object is to provide asemiconductor device with low power consumption. Another object is toprovide a semiconductor device with high reliability. Another object isto provide a semiconductor device which can retain data even when powersupply is stopped. Another object is to provide a method formanufacturing the semiconductor device.

Note that the descriptions of these objects do not disturb the existenceof other objects. Note that in one embodiment of the present invention,there is no need to achieve all the objects. Other objects are apparentfrom and can be derived from the description of the specification, thedrawings, the claims, and the like.

One embodiment of the present invention relates to a semiconductordevice in which a source electrode layer or a drain electrode layer isformed on an upper surface of an oxide semiconductor layer.

Note that “side contact” in this specification means a state where aside surface of one element is in contact with part of the otherelement, so that electrical connection between the one element and theother element is obtained.

One embodiment of the present invention is a semiconductor deviceincluding a first oxide semiconductor layer over an insulating surface;a second oxide semiconductor layer over the first oxide semiconductorlayer; a source electrode layer and a drain electrode layer which areover the second oxide semiconductor layer and whose side surfaces areprovided on the same surface as side surfaces of the second oxidesemiconductor layer; a third oxide semiconductor layer which is over thesecond oxide semiconductor layer and partly in contact with each of thesource electrode layer and the drain electrode layer; a gate insulatingfilm over the third oxide semiconductor layer; a gate electrode layerover the gate insulating film; and an insulating layer over theinsulating surface, the source electrode layer, the drain electrodelayer, and the gate electrode layer. In the insulating layer, a firstopening where part of the second oxide semiconductor layer and part ofthe source electrode layer are exposed, a second opening where part ofthe second oxide semiconductor layer and part of the drain electrodelayer are exposed, and a third opening where part of the gate electrodelayer is exposed are formed. The second oxide semiconductor layer andthe source electrode layer are electrically connected to a first wiringin the first opening. The second oxide semiconductor layer and the drainelectrode layer are electrically connected to a second wiring in thesecond opening. The gate electrode layer is electrically connected to athird wiring in the third opening.

Note that in this specification and the like, ordinal numbers such as“first” and “second” are used in order to avoid confusion amongcomponents and do not limit the components numerically.

Further, a conduction band minimum of the first oxide semiconductorlayer and a conduction band minimum of the third oxide semiconductorlayer are preferably closer to a vacuum level than a conduction bandminimum of the second oxide semiconductor layer by 0.05 eV or more and 2eV or less.

It is preferable that the first oxide semiconductor layer, the secondoxide semiconductor layer, and the third oxide semiconductor layer eachinclude an In-M-Zn oxide (M is Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf),and that an atomic ratio of M to In in each of the first oxidesemiconductor layer and the third oxide semiconductor layer be higherthan an atomic ratio of M to In in the second oxide semiconductor layer.

Each of the first oxide semiconductor layer, the second oxidesemiconductor layer, and the third oxide semiconductor layer preferablyincludes a crystal in which c-axes are aligned.

Further, the source electrode layer and the drain electrode layer areeach preferably formed of a single layer of Al, Cr, Cu, Ta, Ti, Mo, orW, a stacked film of any of these, or an alloy material containing anyof these as its main component.

Another embodiment of the present invention is a method formanufacturing a semiconductor device comprising the steps of: forming astacked film of a first oxide semiconductor film and a second oxidesemiconductor film over an insulating surface; forming a firstconductive film over the stacked film; forming a first resist mask overthe first conductive film; selectively etching the first conductive filmusing the first resist mask as a mask to form a first conductive layer;selectively etching the stacked film using the first conductive layer asa mask and selectively etching the first conductive layer to divide thefirst conductive layer, thereby forming a stack of a first oxidesemiconductor layer and a second oxide semiconductor layer and a sourceelectrode layer and a drain electrode layer over the stack; forming athird oxide semiconductor film over the insulating surface, the stack,the source electrode layer, and the drain electrode layer; forming anoxide insulating film over the third oxide semiconductor film; forming asecond conductive film over the oxide insulating film; forming a secondresist mask over the second conductive film; selectively etching thesecond conductive film using the second resist mask as a mask to form agate electrode layer; selectively etching the oxide insulating film andthe third oxide semiconductor film using the gate electrode layer as amask to form a gate insulating film and a third oxide semiconductorlayer; forming an insulating layer over the insulating surface, thesource electrode layer, the drain electrode layer, and the gateelectrode layer; forming, in the insulating layer, a first opening wherepart of the second oxide semiconductor layer and part of the sourceelectrode layer are exposed, a second opening where part of the secondoxide semiconductor layer and part of the drain electrode layer areexposed, and a third opening where part of the gate electrode layer isexposed; and forming a first wiring electrically connected to the secondoxide semiconductor layer and the source electrode layer in the firstopening, a second wiring electrically connected to the second oxidesemiconductor layer and the drain electrode layer in the second opening,and a third wiring electrically connected to the gate electrode layer inthe third opening.

Further, the first oxide semiconductor layer and the third oxidesemiconductor layer are each preferably formed using a material in whicha conduction band minimum of the first oxide semiconductor layer and aconduction band minimum of the third oxide semiconductor layer arecloser to a vacuum level than a conduction band minimum of the secondoxide semiconductor layer is by 0.05 eV or more and 2 eV or less.

The first oxide semiconductor layer, the second oxide semiconductorlayer, and the third oxide semiconductor layer are each preferablyformed using an In-M-Zn oxide (M is Al, Ti, Ga, Y, Zr, La, Ce, Nd, orHf), and that an atomic ratio of M to In in each of the first oxidesemiconductor layer and the third oxide semiconductor layer be higherthan an atomic ratio of M to In in the second oxide semiconductor layer.

For each of the first oxide semiconductor layer, the second oxidesemiconductor layer, and the third oxide semiconductor layer, a materialincluding a crystal in which c-axes are aligned is preferably used.

In the above structure, it is preferable that the source electrode layerand the drain electrode layer be each formed using a single layer of Al,Cr, Cu, Ta, Ti, Mo, or W, a stacked layer of any of these, or an alloymaterial containing any of these as its main component.

According to one embodiment of the present invention, a semiconductordevice having a structure with which the device can be manufactured in asimple process even when the device is miniaturized can be provided.Alternatively, a semiconductor device having a structure which canprevent a decrease in yield caused by miniaturization can be provided.Alternatively, a semiconductor device in which a deterioration inelectrical characteristics which becomes more noticeable as thetransistor is miniaturized can be suppressed can be provided.Alternatively, a highly integrated semiconductor device can be provided.Alternatively, a semiconductor device in which deterioration inelectrical characteristics is reduced can be provided. Alternatively, asemiconductor device in which variation in electrical characteristics issuppressed can be provided. Alternatively, a semiconductor device withlow power consumption can be provided. Alternatively, a highly reliablesemiconductor device can be provided. Alternatively, a semiconductordevice in which data is retained even when not powered can be provided.Alternatively, a method for manufacturing the above semiconductor devicecan be provided.

Note that the descriptions of these effects do not disturb the existenceof other effects. In one embodiment of the present invention, there isno need to obtain all the effects. Other effects are apparent from andcan be derived from the description of the specification, the drawings,the claims, and the like.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are a top view and a cross-sectional view of atransistor;

FIGS. 2A and 2B are cross-sectional views of a transistor;

FIGS. 3A to 3C are cross-sectional views of transistors;

FIGS. 4A to 4C are cross-sectional views of transistors;

FIG. 5 is a cross-sectional view of a transistor;

FIGS. 6A and 6B are cross-sectional views of transistors;

FIG. 7 is a cross-sectional view of a transistor;

FIGS. 8A to 8C are cross-sectional views illustrating a method formanufacturing a transistor;

FIGS. 9A to 9C are cross-sectional views illustrating a method formanufacturing a transistor;

FIGS. 10A and 10B are cross-sectional views illustrating a method formanufacturing a transistor;

FIGS. 11A and 11B are a cross-sectional view and a circuit diagram of asemiconductor device;

FIG. 12 is a circuit diagram of a semiconductor device;

FIGS. 13A to 13C illustrate electronic devices to which semiconductordevices can be applied;

FIGS. 14A and 14B are a top view and a cross-sectional view of atransistor;

FIGS. 15A and 15B are a top view and a cross-sectional view of atransistor;

FIGS. 16A to 16D show models used for calculation and calculationresults thereof; and

FIGS. 17A and 17B show Id-Vg characteristics of a transistor.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments are described in detail with reference to the drawings. Notethat the present invention is not limited to the following descriptionand it is readily appreciated by those skilled in the art that modes anddetails can be modified in various ways without departing from thespirit and the scope of the present invention. Therefore, the presentinvention should not be limited to the descriptions of the embodimentsbelow. Note that in structures of the present invention described below,the same portions or portions having similar functions are denoted bythe same reference numerals in different drawings, and descriptionthereof is omitted in some cases.

Note that in this specification and the like, when it is explicitlydescribed that X and Y are connected, the case where X and Y areelectrically connected, the case where X and Y are functionallyconnected, and the case where X and Y are directly connected areincluded therein. Here, each of X and Y denotes an object (e.g., adevice, an element, a circuit, a wiring, an electrode, a terminal, aconductive film, a layer, or the like). Accordingly, a connectionrelation other than connection relations shown in the drawings and textsis also included, without being limited to a predetermined connectionrelation, for example, a connection relation shown in the drawings andtexts.

In the case where X and Y are electrically connected, one or moreelements (e.g., a switch, a transistor, a capacitor, an inductor, aresistor, a diode, a display element, a light-emitting element, and aload) that enable an electrical connection between X and Y can beconnected between X and Y, for example. Note that the switch iscontrolled to be turned on or off. That is, the switch has a function ofdetermining whether current flows or not by being turned on or off(becoming an on state and an off state). Alternatively, the switch has afunction of selecting and changing a current path.

In the case where X and Y are functionally connected, one or morecircuits (e.g., a logic circuit such as an inverter, a NAND circuit, ora NOR circuit; a signal converter circuit such as a DA convertercircuit, an AD converter circuit, or a gamma correction circuit; apotential level converter circuit such as a power supply circuit (e.g.,a step-up circuit or a step-down circuit) or a level shifter circuit forchanging the potential level of a signal; a voltage source; a currentsource; a switching circuit; an amplifier circuit such as a circuit thatcan increase signal amplitude, the amount of current, or the like, anoperational amplifier, a differential amplifier circuit, a sourcefollower circuit, or a buffer circuit; a signal generation circuit; astorage circuit; and a control circuit) that enable a functionalconnection between X and Y can be connected between X and Y, forexample. Note that for example, in the case where a signal output from Xis transmitted to Y even when another circuit is interposed between Xand Y, X and Y are functionally connected.

Note that when it is explicitly described that X and Y are connected,the case where X and Y are electrically connected (i.e., the case whereX and Y are connected with another element or another circuit providedtherebetween), the case where X and Y are functionally connected (i.e.,the case where X and Y are functionally connected with another circuitprovided therebetween), and the case where X and Y are directlyconnected (i.e., the case where X and Y are connected without anotherelement or another circuit provided therebetween) are included therein.That is, when it is explicitly described that “X and Y are electricallyconnected”, the description is the same as the case where it isexplicitly only described that “X and Y are connected”.

Even when independent components are electrically connected to eachother in a circuit diagram, one component has functions of a pluralityof components in some cases. For example, when part of a wiring alsofunctions as an electrode, one conductive film functions as the wiringand the electrode. Thus, an “electrical connection” in thisspecification includes in its category such a case where one conductivefilm has functions of a plurality of components.

Note that in this specification and the like, a transistor can be formedusing any of a variety of substrates. The type of a substrate is notlimited to a certain type. Examples of the substrate include asemiconductor substrate (e.g., a single crystal substrate or a siliconsubstrate), an SOI substrate, a glass substrate, a quartz substrate, aplastic substrate, a metal substrate, a stainless steel substrate, asubstrate including stainless steel foil, a tungsten substrate, asubstrate including tungsten foil, a flexible substrate, an attachmentfilm, paper including a fibrous material, and a base material film.Examples of a glass substrate include a barium borosilicate glasssubstrate, an aluminoborosilicate glass substrate, and a soda lime glasssubstrate. For a flexible substrate, a flexible synthetic resin such asplastics typified by polyethylene terephthalate (PET), polyethylenenaphthalate (PEN), and polyether sulfone (PES), or acrylic can be used,for example. Examples of an attachment film include attachment filmsformed using polypropylene, polyester, polyvinyl fluoride, polyvinylchloride, and the like. Examples of a base film include a base filmformed using polyester, polyamide, polyimide, an inorganic vapordeposition film, paper, and the like. Specifically, when a transistor isformed using a semiconductor substrate, a single crystal substrate, anSOI substrate, or the like, a transistor with few variations incharacteristics, size, shape, or the like, high current supplycapability, and a small size can be formed. By forming a circuit usingsuch a transistor, power consumption of the circuit can be reduced orthe circuit can be highly integrated.

Note that a transistor may be formed using one substrate, and then, thetransistor may be transferred to another substrate. Examples of asubstrate to which a transistor is transferred include, in addition tothe above-described substrates over which transistors can be formed, apaper substrate, a cellophane substrate, a stone substrate, a woodsubstrate, a cloth substrate (including a natural fiber (e.g., silk,cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, orpolyester), a regenerated fiber (e.g., acetate, cupra, rayon, orregenerated polyester), or the like), a leather substrate, a rubbersubstrate, and the like. With the use of such a substrate, a transistorwith excellent properties, a transistor with low power consumption, or adevice with high durability can be formed, high heat resistance can beprovided, or a reduction in weight or thinning can be achieved.

Embodiment 1

In this embodiment, a semiconductor device of one embodiment of thepresent invention is described with reference to drawings.

FIGS. 1A and 1B are a top view and a cross-sectional view of atransistor of one embodiment of the present invention. FIG. 1A is thetop view. FIG. 1B illustrates a cross section taken along dashed-dottedline A1-A2 in. FIG. 1A. FIG. 2A is a cross-sectional view taken alongdashed-dotted line A3-A4 in FIG. 1A. FIG. 2B is a cross-sectional viewtaken along dashed-dotted line A5-A6 in FIG. 1A. Note that forsimplification of the drawing, some components are not illustrated inthe top view in FIG. 1A. In some cases, the direction of thedashed-dotted line A1-A2 is referred to as a channel length direction,and the direction of the dashed-dotted line A3-A4 is referred to as achannel width direction.

A transistor 100 illustrated in FIGS. 1A and 1B and FIGS. 2A and 2Bincludes a base insulating film 120 over a substrate 110, a stack inwhich a first oxide semiconductor layer 131 and a second oxidesemiconductor layer 132 are formed in this order and which is over thebase insulating film, a source electrode layer 140 and a drain electrodelayer 150 over the second oxide semiconductor layer, a third oxidesemiconductor layer 133 which is formed in contact with the baseinsulating film 120 and the stack and is partly in contact with each ofthe source electrode layer 140 and the drain electrode layer 150, a gateinsulating film 160 over the third oxide semiconductor layer, a gateelectrode layer 170 over the gate insulating film, and an insulatinglayer 180 over the base insulating film 120, the source electrode layer140, the drain electrode layer 150, and the gate electrode layer 170.

Note that functions of a “source” and a “drain” of a transistor aresometimes replaced with each other when a transistor of oppositepolarity is used or when the direction of current flowing is changed incircuit operation, for example. Thus, the terms “source” and “drain” canbe used to denote the drain and the source, respectively, in thisspecification.

An insulating layer 185 formed of an oxide may be formed over theinsulating layer 180. Note that the insulating layer 185 may be providedas needed and another insulating layer may be further providedthereover. The first oxide semiconductor layer 131, the second oxidesemiconductor layer 132, and the third oxide semiconductor layer 133 arecollectively referred to as an oxide semiconductor layer 130.

In the insulating layer 180, a first opening 147 where the second oxidesemiconductor layer 132 and the source electrode layer 140 are partlyexposed is formed. Further, a second opening 157 where the second oxidesemiconductor layer 132 and the drain electrode layer 150 are partlyexposed is formed. Furthermore, a third opening 177 where the gateelectrode layer 170 is partly exposed is formed.

In the first opening 147, a side surface of the second oxidesemiconductor layer 132 and a side surface of the source electrode layer140 are provided on the same surface and are electrically connected to afirst wiring 145. In the second opening 157, a side surface of thesecond oxide semiconductor layer 132 and a side surface of the drainelectrode layer 150 are provided on the same surface and areelectrically connected to a second wiring 155. In the third opening 177,the gate electrode layer 170 is electrically connected to a third wiring175 by side contact.

Conventionally, electrical connection has been obtained by providing anopening in an insulating layer and the like formed on an electrode layerso that part of a wiring formed in the opening is in contact with partof the upper surface of the electrode layer.

However, as miniaturization of the transistor progresses, the degree ofdifficulty in manufacturing increases, which results in a defect in theopening provided in the insulating layer or the like, variation in thedepth of the opening, and the like. Thus, contact resistance between theelectrode layer and the wiring is likely to vary among elements. Inother words, an increase in the degree of difficulty in manufacturing aminiaturized transistor is one factor of variation in the electricalcharacteristics of transistors.

On the other hand, in one embodiment of the present invention, part ofan electrode layer exposed in an opening and part of a wiring formed inthe opening are electrically connected to each other by side contact.Thus, variation in contact area between the electrode layer and thewiring can be less likely to occur. In other words, variation in contactresistance between the electrode layer and the wiring in elements can besuppressed, which enables reduction in variation in the electricalcharacteristics of a transistor which is caused by the variation.

Further, in the case where an opening is provided in an insulating layerto expose an electrode layer and the like, over-etching an electrodelayer and the like to expose side surfaces of the electrode layer andthe like in the opening is less difficult than exposing upper surfacesof the electrode layer and the like by controlling etching conditionsstrictly. In the case where an opening is formed so as to extend intothe electrode layer, for example, the etching conditions can be freelyselected even when the etching rate of the electrode layer issufficiently lower than that of the insulating layer. Accordingly, theyield of the transistor can be improved.

In one embodiment of the present invention, it is preferable to employ astructure in which the first opening 147 and the second opening 157reach the base insulating film 120 as illustrated in FIG. 1B. Thestructure can be formed under etching conditions having a high degree offreedom and can reduce variation in electrical characteristics of atransistor and improve yield. Moreover, since a wiring in contact with asemiconductor layer functions as part of an electrode layer, contactresistance between the electrode layer and the semiconductor layer canbe further reduced.

Further, when the gate electrode layer 170 is connected to the thirdwiring 175 by side contact as illustrated in FIGS. 2A and 2B, variationin contact area between the electrode layer and the wiring can be lesslikely to occur and variation in contact resistance can be suppressed.Note that the bottom of the third opening 177 is positioned in a range D(in any of the gate insulating film 160, the third oxide semiconductorlayer 133, and the base insulating film 120) in the drawing.

Note that the structures inside the first opening 147 and the secondopening 157 are not limited to the example illustrated in FIG. 1B. Forexample, as illustrated in FIG. 3A, a structure may be employed in whichupper surfaces of the source electrode layer 140 and the drain electrodelayer 150 are partly exposed in the first opening 147 and the secondopening 157. When the etching rates of the source electrode layer 140and the drain electrode layer 150 are sufficiently lower than that ofthe insulating layer 180, the structure can be formed easily.

Alternatively, as illustrated in FIG. 3B, a structure may be employed inwhich an upper surface of the second oxide semiconductor layer 132 ispartly exposed in the first opening 147 and the second opening 157.Further alternatively, although not illustrated, a structure may beemployed in which an upper surface of the first oxide semiconductorlayer 131 is partly exposed in the openings. When the etching rate ofthe second oxide semiconductor layer 132 and/or the etching rate of thefirst oxide semiconductor layer 131 are/is sufficiently lower than thatof the insulating layer 180, the structure can be formed easily.

Note that in the description of FIGS. 3A and 3B, a layer whose uppersurface is partly exposed may be partly etched in the film thicknessdirection.

Still further alternatively, as illustrated in FIG. 3C, the bottoms ofthe first opening 147 and the second opening 157 may be positioned inthe base insulating film 120. When the etching rate of the insulatinglayer 180 is close to the etching rate of each of the source electrodelayer 140, the drain electrode layer 150, the second oxide semiconductorlayer 132, the first oxide semiconductor layer 131, and the baseinsulating film 120, the structure can be formed easily.

Note that when the etching conditions can be controlled strictly, asillustrated in FIGS. 14A and 14B, a structure may be employed in whichupper surfaces of the source electrode layer 140 and the drain electrodelayer 150 are partly exposed to be in contact with the first wiring 145and the second wiring 155.

Alternatively, in a transistor of one embodiment of the presentinvention, as illustrated in FIGS. 15A and 15B, upper surface shapes ofthe third oxide semiconductor layer 133 and the gate insulating film 160may be different from an upper surface shape of the gate electrode layer170. The structure illustrated in FIGS. 15A and 15B can reduce gateleakage current. Note that the structure can be applied to anothertransistor described in this embodiment.

Since the source electrode layer 140 and the drain electrode layer 150are formed only over the oxide semiconductor layer in the transistor ofone embodiment of the present invention, there is a concern that aneffective channel width is shortened and thus the on-state currentdecreases slightly; however, application of gate electric field to aside portion of the oxide semiconductor layer is not blocked and thusgate electric field is applied to the entire oxide semiconductor layer,whereby the S value of the transistor can be decreased. The effect isconfirmed by the scientific calculation described below.

FIG. 16A is a top view of a model (a) assuming a transistor having aconventional structure, and the width of each of the source electrodelayer 140 and the drain electrode layer 150 is larger than that of theoxide semiconductor layer. FIG. 16B is a top view of a model (b)assuming one embodiment of the present invention, and the width of eachof the source electrode layer 140 and the drain electrode layer 150 isthe same as that of the oxide semiconductor layer.

FIG. 16C shows calculation results of current density distributions incross sections of channel portions in the W width direction at a draincurrent of about 1E-12 [A] in the models. The left part of FIG. 16Cshows calculation results of the model (a) and the current density ishigh around the center of a lower layer of the channel portion.

In other words, current cannot be controlled around the center of thelower layer of the channel portion. On the other hand, the right part ofFIG. 16C shows calculation results of the model (b) and the currentdensity is high near an upper layer of the channel portion. This isbecause gate electric field is sufficiently applied from the sidesurface.

As shown in FIG. 16D, from Id-Vg characteristics obtained by calculationusing the above models, it is found that the S value of the model (b)assuming one embodiment of the present invention is extremely small ascompared to the model (a).

Next, components of the transistor 100 of one embodiment of the presentinvention are described in detail.

The substrate 110 is not limited to a simple supporting substrate, andmay be a substrate where another device such as a transistor is formed.In that case, one of the gate electrode layer 170, the source electrodelayer 140, and the drain electrode layer 150 of the transistor 100 maybe electrically connected to the above device.

The base insulating film 120 can have a function of supplying oxygen tothe oxide semiconductor layer 130 as well as a function of preventingdiffusion of impurities from the substrate 110. For this reason, thebase insulating film 120 is preferably an insulating film containingoxygen and further preferably, the base insulating film 120 is aninsulating film containing oxygen in which the oxygen content is higherthan that in the stoichiometric composition. In the case where thesubstrate 110 is provided with another device as described above, thebase insulating film 120 also has a function as an interlayer insulatingfilm. In that case, the base insulating film 120 is preferably subjectedto planarization treatment such as chemical mechanical polishing (CMP)treatment so as to have a flat surface.

Further, in a region where a channel of the transistor 100 is formed,the oxide semiconductor layer 130 has a structure in which the firstoxide semiconductor layer 131, the second oxide semiconductor layer 132,and the third oxide semiconductor layer 133 are stacked in this orderfrom the substrate 110 side. Furthermore, in the first oxidesemiconductor layer 131, a region not overlapping with the second oxidesemiconductor layer 132, the source electrode layer 140, and the drainelectrode layer 150 is in contact with the third oxide semiconductorlayer 133, which means that the second oxide semiconductor layer 132 issurrounded by the first oxide semiconductor layer 131 and the thirdoxide semiconductor layer 133.

Here, for the second oxide semiconductor layer 132, for example, anoxide semiconductor whose electron affinity (an energy differencebetween a vacuum level and the conduction band minimum) is higher thanthose of the first oxide semiconductor layer 131 and the third oxidesemiconductor layer 133 is used. The electron affinity can be obtainedby subtracting an energy difference between the conduction band minimumand the valence band maximum (what is called an energy gap) from anenergy difference between the vacuum level and the valence band maximum(what is called an ionization potential).

Although the case where the oxide semiconductor layer 130 is a stack ofthree layers is described in detail in this embodiment, the oxidesemiconductor layer 130 may be a single layer or a stack of two layersor four or more layers. In the case where the oxide semiconductor layer130 is a single layer, for example, a layer corresponding to the secondoxide semiconductor layer 132 is used as illustrated in FIG. 4A. In thecase where the oxide semiconductor layer 130 is a stack of two layers,for example, a structure without the third oxide semiconductor layer 133is used as illustrated in FIG. 4B. In such a case, the second oxidesemiconductor layer 132 and the first oxide semiconductor layer 131 canbe replaced with each other. In the case where the oxide semiconductorlayer 130 is a stack of three layers, a structure different from that inFIGS. 1A and 1B, such as that in FIG. 4C, can be employed. In the caseof a stack of four or more layers, for example, a structure in which anoxide semiconductor layer is stacked over the three-layer stackedstructure described in this embodiment or a structure in which an oxidesemiconductor layer is provided between any of two layers in thethree-layer stacked structure can be employed.

It is preferable that each of the first oxide semiconductor layer 131and the third oxide semiconductor layer 133 contain one or more kinds ofmetal elements forming the second oxide semiconductor layer 132 and beformed, for example, using an oxide semiconductor whose energy of theconduction band minimum is closer to the vacuum level than that of thesecond oxide semiconductor layer 132 by 0.05 eV or more, 0.07 eV ormore, 0.1 eV or more, or 0.15 eV or more and 2 eV or less, 1 eV or less,0.5 eV or less, or 0.4 eV or less.

In such a structure, when an electric field is applied to the gateelectrode layer 170, a channel is formed in the second oxidesemiconductor layer 132 whose conduction band minimum is the lowest inthe oxide semiconductor layer 130. In other words, the third oxidesemiconductor layer 133 is formed between the second oxide semiconductorlayer 132 and the gate insulating film 160, whereby a structure in whichthe channel of the transistor is not in contact with the gate insulatingfilm 160 is obtained.

Further, since the first oxide semiconductor layer 131 contains one ormore metal elements contained in the second oxide semiconductor layer132, an interface state is less likely to be formed at the interface ofthe second oxide semiconductor layer 132 with the first oxidesemiconductor layer 131 than at the interface with the base insulatingfilm 120 on the assumption that the second oxide semiconductor layer 132is in contact with the base insulating film 120. The interface statesometimes forms a channel, leading to a change in the threshold voltageof the transistor. Thus, with the first oxide semiconductor layer 131,variation in the electrical characteristics of the transistors, such asa threshold voltage, can be reduced. Further, the reliability of thetransistor can be improved.

Furthermore, since the third oxide semiconductor layer 133 contains oneor more metal elements contained in the second oxide semiconductor layer132, scattering of carriers is less likely to occur at the interface ofthe second oxide semiconductor layer 132 with the third oxidesemiconductor layer 133 than at the interface with the gate insulatingfilm 160 on the assumption that the second oxide semiconductor layer 132is in contact with the gate insulating film 160. Thus, with the thirdoxide semiconductor layer 133, the field-effect mobility of thetransistor can be increased.

For the first oxide semiconductor layer 131 and the third oxidesemiconductor layer 133, for example, a material containing Al, Ti, Ga,Ge, Y, Zr, Sn, La, Ce, or Hf with a higher atomic ratio than that usedfor the second oxide semiconductor layer 132 can be used. Specifically,an atomic ratio of any of the above metal elements in the first oxidesemiconductor layer 131 and the third oxide semiconductor layer 133 is1.5 times or more, preferably 2 times or more, further preferably 3times or more as much as that in the second oxide semiconductor layer132. Any of the above elements is strongly bonded to oxygen and thus hasa function of suppressing generation of an oxygen vacancy in an oxidesemiconductor layer. That is, an oxygen vacancy is less likely to begenerated in the first oxide semiconductor layer 131 and the third oxidesemiconductor layer 133 than in the second oxide semiconductor layer132.

Note that when each of the first oxide semiconductor layer 131, thesecond oxide semiconductor layer 132, and the third oxide semiconductorlayer 133 is an In-M-Zn oxide containing at least indium, zinc, and M (Mis a metal such as Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf), and thefirst oxide semiconductor layer 131 has an atomic ratio of In to M andZn which is x₁:y₁:z₁, the second oxide semiconductor layer 132 has anatomic ratio of In to M and Zn which is x₂:y₂:z₂, and the third oxidesemiconductor layer 133 has an atomic ratio of In to M and Zn which isx₃:y₃:z₃, each of y₁/x₁ and y₃/x₃ is preferably larger than y₂/x₂. Eachof y₁/x₁ and y₃/x₃ is 1.5 times or more, preferably 2 times or more,further preferably 3 times or more as large as y₂/x₂. At this time, wheny₂ is greater than or equal to x₂ in the second oxide semiconductorlayer 132, the transistor can have stable electrical characteristics.However, when y₂ is 3 times or more as large as x₂, the field-effectmobility of the transistor is reduced; accordingly, y₂ is preferablyless than 3 times x₂.

Note that in this specification, an atomic ratio used for describing thecomposition of an oxide semiconductor layer can be also used as theatomic ratio of a base material. In the case where an oxidesemiconductor layer is deposited by a sputtering method using an oxidesemiconductor material as a target, the composition of the oxidesemiconductor layer might be different from that of the target, which isa base material, depending on the kind or a ratio of a sputtering gas,the density of the target, or deposition conditions. Thus, in thisspecification, an atomic ratio used for describing the composition of anoxide semiconductor layer is also used as the atomic ratio of a basematerial. For example, in the case where a sputtering method is used fordeposition, an In—Ga—Zn oxide film whose atomic ratio of In to Ga and Znis 1:1:1 can be also understood as an In—Ga—Zn oxide film formed usingan In—Ga—Zn oxide material whose atomic ratio of In to Ga and Zn is1:1:1 as a target.

Further, in the case where Zn and O are not taken into consideration,the proportion of In and the proportion of M in each of the first oxidesemiconductor layer 131 and the third oxide semiconductor layer 133 arepreferably less than 50 atomic % and greater than or equal to 50 atomic%, respectively, and further preferably less than 25 atomic % andgreater than or equal to 75 atomic %, respectively. In addition, in thecase where Zn and O are not taken into consideration, the proportion ofIn and the proportion of M in the second oxide semiconductor layer 132are preferably greater than or equal to 25 atomic % and less than 75atomic %, respectively, and further preferably greater than or equal to34 atomic % and less than 66 atomic %, respectively.

The thicknesses of the first oxide semiconductor layer 131 and the thirdoxide semiconductor layer 133 are each greater than or equal to 1 nm andless than or equal to 100 nm, preferably greater than or equal to 3 nmand less than or equal to 50 nm. The thickness of the second oxidesemiconductor layer 132 is greater than or equal to 1 nm and less thanor equal to 200 nm, preferably greater than or equal to 3 nm and lessthan or equal to 100 nm, further preferably greater than or equal to 3nm and less than or equal to 50 nm.

For each of the first oxide semiconductor layer 131, the second oxidesemiconductor layer 132, and the third oxide semiconductor layer 133, anoxide semiconductor containing indium, zinc, and gallium can be used,for example. Note that the second oxide semiconductor layer 132preferably contains indium because carrier mobility can be increased.

Note that stable electrical characteristics can be effectively impartedto a transistor in which an oxide semiconductor layer serves as achannel by reducing the concentration of impurities in the oxidesemiconductor layer to make the oxide semiconductor layer intrinsic orsubstantially intrinsic. The term “substantially intrinsic” refers tothe state where an oxide semiconductor layer has a carrier density lowerthan 1×10¹⁷/cm³, preferably lower than 1×10¹⁵/cm³, further preferablylower than 1×10¹³/cm³.

Further, in the oxide semiconductor layer, hydrogen, nitrogen, carbon,silicon, and a metal element other than main components are impurities.For example, hydrogen and nitrogen form donor levels to increase thecarrier density, and silicon forms impurity levels in the oxidesemiconductor layer. The impurity levels serve as traps and might causethe electrical characteristics of the transistor to deteriorate. Thus,it is preferable to reduce the concentration of the impurities in thefirst oxide semiconductor layer 131, the second oxide semiconductorlayer 132, and the third oxide semiconductor layer 133, and atinterfaces between the layers.

In order to make the oxide semiconductor layer intrinsic orsubstantially intrinsic, in SIMS (secondary ion mass spectrometry), forexample, the concentration of silicon at a certain depth of the oxidesemiconductor layer or in a region of the oxide semiconductor layer ispreferably lower than 1×10¹⁹ atoms/cm³, further preferably lower than5×10¹⁸ atoms/cm³, still further preferably lower than 1×10¹⁸ atoms/cm³.Further, the concentration of hydrogen at a certain depth of the oxidesemiconductor layer or in a region of the oxide semiconductor layer ispreferably lower than or equal to 2×10²⁰ atoms/cm³, further preferablylower than or equal to 5×10¹⁹ atoms/cm³, still further preferably lowerthan or equal to 1×10¹⁹ atoms/cm³, yet still further preferably lowerthan or equal to 5×10¹⁸ atoms/cm³. Further, the concentration ofnitrogen at a certain depth of the oxide semiconductor layer or in aregion of the oxide semiconductor layer is preferably lower than 5×10¹⁹atoms/cm³, further preferably lower than or equal to 5×10¹⁸ atoms/cm³,still further preferably lower than or equal to 1×10¹⁸ atoms/cm³, yetstill further preferably lower than or equal to 5×10¹⁷ atoms/cm³.

In the case where the oxide semiconductor layer includes crystals, highconcentration of silicon or carbon might reduce the crystallinity of theoxide semiconductor layer. In order not to lower the crystallinity ofthe oxide semiconductor layer, for example, the concentration of siliconat a certain depth of the oxide semiconductor layer or in a region ofthe oxide semiconductor layer may be lower than 1×10¹⁹ atoms/cm³,preferably lower than 5×10¹⁸ atoms/cm³, further preferably lower than1×10¹⁸ atoms/cm³. Further, the concentration of carbon at a certaindepth of the oxide semiconductor layer or in a region of the oxidesemiconductor layer may be lower than 1×10¹⁹ atoms/cm³, preferably lowerthan 5×10¹⁸ atoms/cm³, further preferably lower than 1×10¹⁸ atoms/cm³,for example.

A transistor in which the above-described highly purified oxidesemiconductor layer is used for a channel formation region has anextremely low off-state current. In the case where the voltage between asource and a drain is set to about 0.1 V, 5 V, or 10 V, for example, theoff-state current standardized on the channel width of the transistorcan be as low as several yoctoamperes per micrometer to severalzeptoamperes per micrometer.

Note that as the gate insulating film of the transistor, an insulatingfilm containing silicon is used in many cases; thus, it is preferablethat, as in the transistor of one embodiment of the present invention, aregion of the oxide semiconductor layer, which serves as a channel, benot in contact with the gate insulating film for the above-describedreason. In the case where a channel is formed at the interface betweenthe gate insulating film and the oxide semiconductor layer, scatteringof carriers occurs at the interface, whereby the field-effect mobilityof the transistor is reduced in some cases. Also from the view of theabove, it is preferable that the region of the oxide semiconductorlayer, which serves as a channel, be separated from the gate insulatingfilm.

Accordingly, with the oxide semiconductor layer 130 having astacked-layer structure including the first oxide semiconductor layer131, the second oxide semiconductor layer 132, and the third oxidesemiconductor layer 133, a channel can be formed in the second oxidesemiconductor layer 132; thus, the transistor can have a highfield-effect mobility and stable electrical characteristics.

In the band structures of the first oxide semiconductor layer 131, thesecond oxide semiconductor layer 132, and the third oxide semiconductorlayer 133, the energy of the conduction band minimum continuouslychanges. This can be understood also from the fact that the compositionsof the first oxide semiconductor layer 131, the second oxidesemiconductor layer 132, and the third oxide semiconductor layer 133 areclose to one another and oxygen is easily diffused among the first oxidesemiconductor layer 131, the second oxide semiconductor layer 132, andthe third oxide semiconductor layer 133. Thus, the first oxidesemiconductor layer 131, the second oxide semiconductor layer 132, andthe third oxide semiconductor layer 133 have a continuous physicalproperty although they have different compositions and form a stack. Inthe drawings, interfaces between the oxide semiconductor layers of thestack are indicated by dotted lines.

The oxide semiconductor layer 130 in which layers containing the samemain components are stacked is formed to have not only a simplestacked-layer structure of the layers but also a continuous junction(here, in particular, a well structure having a U shape in whichenergies of the conduction band minimums successively vary betweenlayers). In other words, the stacked-layer structure is formed such thatthere exists no impurity that forms a defect level such as a trap centeror a recombination center at each interface. If impurities exist betweenthe stacked oxide semiconductor layers, the continuity of the energyband is lost and carriers disappear by a trap or recombination.

For example, an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is1:3:2, 1:3:3, 1:3:4, 1:3:6, 1:6:4, or 1:9:6 can be used for the firstoxide semiconductor layer 131 and the third oxide semiconductor layer133 and an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is1:1:1, 5:5:6, or 3:1:2 can be used for the second oxide semiconductorlayer 132.

The second oxide semiconductor layer 132 of the oxide semiconductorlayer 130 serves as a well, so that a channel is formed in the secondoxide semiconductor layer 132 in a transistor including the oxidesemiconductor layer 130. Note that since the energies of the conductionband minimums continuously changes, the oxide semiconductor layer 130can also be referred to as a U-shaped well. Further, a channel formed tohave such a structure can also be referred to as a buried channel.

Note that trap levels due to impurities or defects might be formed inthe vicinity of the interface between an insulating film such as asilicon oxide film and each of the first oxide semiconductor layer 131and the third oxide semiconductor layer 133. The second oxidesemiconductor layer 132 can be distanced away from the trap levels owingto existence of the first oxide semiconductor layer 131 and the thirdoxide semiconductor layer 133.

However, when the energy difference between the conduction band minimumof the first oxide semiconductor layer 131 and the conduction bandminimum of the second oxide semiconductor layer 132 and the energydifference between the conduction band minimum of the third oxidesemiconductor layer 133 and the conduction band minimum of the secondoxide semiconductor layer 132 are small, an electron in the second oxidesemiconductor layer 132 might reach the trap level by passing over theenergy differences. When the electron is trapped in the trap level, anegative fixed charge is generated at the interface with the insulatingfilm, whereby the threshold voltage of the transistor is shifted in thepositive direction.

Thus, to reduce fluctuations in the threshold voltage of the transistor,energy differences of at least certain values between the conductionband minimum of the second oxide semiconductor layer 132 and theconduction band minimum of each of the first oxide semiconductor layer131 and the third oxide semiconductor layer 133 is necessary. Each ofthe energy differences is preferably greater than or equal to 0.1 eV,further preferably greater than or equal to 0.15 eV.

Note that the first oxide semiconductor layer 131, the second oxidesemiconductor layer 132, and the third oxide semiconductor layer 133preferably include crystal parts. In particular, when crystals in whichc-axes are aligned are used, the transistor can have stable electricalcharacteristics.

In the case where an In—Ga—Zn oxide is used for the oxide semiconductorlayer 130, it is preferable that the third oxide semiconductor layer 133contain less In than the second oxide semiconductor layer 132 so thatdiffusion of In to the gate insulating film is prevented.

For the source electrode layer 140, the drain electrode layer 150, thefirst wiring 145, the second wiring 155, and the third wiring 175, aconductive material which is easily bonded to oxygen is preferably used.For example, Al, Cr, Cu, Ta, Ti, Mo, or W can be used. Among thematerials, it is particularly preferable to use Ti, which is easilybonded to oxygen, or W, which has a high melting point and thus allowssubsequent process temperatures to be relatively high. Note that theconductive material which is easily bonded to oxygen includes, in itscategory, a material to which oxygen is easily diffused. Note that thefirst wiring 145, the second wiring 155, and the third wiring 175 mayeach be a stack such as Ti/Al/Ti.

In addition, a conductive material which is not easily bonded to oxygenmay be used as needed. For example, it is possible to use a single layerformed of a material containing tantalum nitride, titanium nitride,gold, platinum, palladium, or ruthenium or a stack of the conductivematerial and the above conductive material which is easily bonded tooxygen.

When the conductive material which is easily bonded to oxygen is incontact with an oxide semiconductor layer, a phenomenon occurs in whichoxygen in the oxide semiconductor layer is diffused into the conductivematerial which is easily bonded to oxygen. The phenomenon noticeablyoccurs when the temperature is high. Thus, by a heat treatment step inthe manufacturing process of the transistor, oxygen vacancies aregenerated in a region of the oxide semiconductor layer, which is in thevicinity of the interface between the oxide semiconductor layer and eachof the source electrode layer and the drain electrode layer. The oxygenvacancies are bonded to hydrogen slightly contained in the film, wherebythe region is likely changed to an n-type. Thus, the n-type region canserve as a source or a drain of the transistor.

The n-type region is illustrated in an enlarged cross-sectional view ofthe transistor (showing part of a cross section in the channel lengthdirection, which is near the source electrode layer 140) in FIG. 5. Aboundary 135 indicated by a dotted line in the first oxide semiconductorlayer 131 and the second oxide semiconductor layer 132 is a boundarybetween an intrinsic semiconductor region and an n-type semiconductorregion. In the first oxide semiconductor layer 131 and the second oxidesemiconductor layer 132, a region near the source electrode layer 140and the first wiring 145 becomes an n-type region. The boundary 135 isschematically illustrated here, but actually, the boundary is notclearly seen in some cases. Although FIG. 5 shows that part of theboundary 135 extends in the lateral direction in the second oxidesemiconductor layer 132, a region in the first oxide semiconductor layer131 and the second oxide semiconductor layer 132, which is sandwichedbetween the source electrode layer 140 and the base insulating film 120,becomes n-type entirely in the thickness direction, in some cases.

In one embodiment of the present invention, the first wiring 145 and thesecond wiring 155 are connected to the first oxide semiconductor layer131 and the second oxide semiconductor layer 132 by side contact, ann-type region formed in the first oxide semiconductor layer 131 and thesecond oxide semiconductor layer 132 can be enlarged. The n-type regionserves as a source (or a drain) of the transistor. When the n-typeregion is enlarged, the series resistance between a channel formationregion and the source electrode (or the drain electrode) or between thechannel formation region and the first wiring 145 (or the second wiring155) can be reduced and the electrical characteristics of the transistorcan be improved.

In the case of forming a transistor with an extremely short channellength, an n-type region which is formed by the generation of oxygenvacancies might extend in the channel length direction of thetransistor. In that case, the electrical characteristics of thetransistor change; for example, the threshold voltage is shifted, or onand off states of the transistor is hard to control with the gatevoltage (in which case the transistor is turned on). Accordingly, when atransistor with an extremely short channel length is formed, it is notalways preferable that a conductive material easily bonded to oxygen beused for a source electrode layer and a drain electrode layer.

In such a case, a conductive material which is less likely to be bondedto oxygen than the above material can be used for the source electrodelayer 140 and the drain electrode layer 150. As the conductive materialwhich is not easily bonded to oxygen, for example, a material containingtantalum nitride, titanium nitride, gold, platinum, palladium, orruthenium or the like can be used. Note that in the case where theconductive material is in contact with the second oxide semiconductorlayer 132, the source electrode layer 140 and the drain electrode layer150 may each have a structure in which the conductive material which isnot easily bonded to oxygen and the above-described conductive materialthat is easily bonded to oxygen are stacked.

The gate insulating film 160 can be formed using an insulating filmcontaining one or more of aluminum oxide, magnesium oxide, siliconoxide, silicon oxynitride, silicon nitride oxide, silicon nitride,gallium oxide, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. Thegate insulating film 160 may be a stack including any of the abovematerials.

For the gate electrode layer 170, a conductive film formed using Al, Ti,Cr, Co, Ni, Cu, Y, Zr, Mo, Ru, Ag, Ta, W, or the like can be used. Thegate electrode layer may be a stack including any of the abovematerials. Alternatively, a conductive film containing nitrogen may beused for the gate electrode layer.

An aluminum oxide film is preferably contained in the gate insulatingfilm 160 and the insulating layer 180 over the gate electrode layer 170.The aluminum oxide film has a high shielding effect (blocking effect) ofpreventing penetration of both oxygen and impurities such as hydrogenand moisture. Accordingly, the aluminum oxide film can be suitably usedas a protective film that prevents entry of an impurity such as hydrogenor moisture, which causes variation in the electrical characteristics ofthe transistor, into the oxide semiconductor layer 130, release ofoxygen, which is a main component material of the oxide semiconductorlayer 130, from the oxide semiconductor layer during and after themanufacturing process of the transistor, and unnecessary release ofoxygen from the base insulating film 120. Further, oxygen contained inthe aluminum oxide film can be diffused into the oxide semiconductorlayer.

Further, the insulating layer 185 is preferably formed over theinsulating layer 180. The insulating layer 185 can be formed using aninsulating film containing one or more of magnesium oxide, siliconoxide, silicon oxynitride, silicon nitride oxide, silicon nitride,gallium oxide, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. Theinsulating layer 185 may be a stack including any of the abovematerials.

Here, the insulating layer 185 preferably contains excess oxygen. Aninsulating layer containing excess oxygen refers to an insulating layerfrom which oxygen can be released by heat treatment or the like. Theinsulating layer containing excess oxygen is preferably a film in whichthe amount of released oxygen when converted into oxygen atoms is1.0×10¹⁹ atoms/cm³ or more in thermal desorption spectroscopy analysis.Oxygen released from the insulating layer can be diffused into thechannel formation region in the oxide semiconductor layer 130 throughthe gate insulating film 160, so that oxygen vacancies formed in thechannel formation region can be filled with the oxygen. In this manner,the electrical characteristics of the transistor can be stable.

High integration of a semiconductor device requires miniaturization of atransistor. However, it is known that miniaturization of a transistorcauses deterioration of the electrical characteristics of thetransistor. In particular, a reduction in on-state current, which isdirectly caused by a decrease in channel width, is significant.

However, in the transistor of one embodiment of the present invention,as described above, the third oxide semiconductor layer 133 is formed soas to cover the second oxide semiconductor layer 132 where a channel isformed and the channel formation layer and the gate insulating film arenot in contact with each other. Accordingly, scattering of carriers atthe interface between the second oxide semiconductor layer 132 where achannel is formed and the gate insulating film can be reduced and thefield-effect mobility of the transistor can be increased.

The transistor of one embodiment of the present invention can haveparticular improved electrical characteristics when having a structurein which the length (W_(T)) of a top surface of the second oxidesemiconductor layer 132 in the channel width direction is as large as orsmaller than the thickness of the oxide semiconductor layer, as in across-sectional diagram in the channel width direction in FIGS. 6A and6B. Note that in the cross section in the channel width direction, thesecond oxide semiconductor layer 132 may have tapered side surfaces andan upper surface having a flat portion as illustrated in FIG. 6A.Alternatively, as illustrated in FIG. 6B, the second oxide semiconductorlayer 132 may have tapered side surfaces and an upper surface having acurvature.

In the case where W_(T) is sufficiently small as in either of thetransistors illustrated in FIGS. 6A and 6B, for example, an electricfield from the gate electrode layer 170 to the side surface of thesecond oxide semiconductor layer 132 is applied to the entire secondoxide semiconductor layer 132; thus, a channel is formed equally in theside and top surfaces of the second oxide semiconductor layer 132.

In the case where a channel region 137 as in either of FIGS. 6A and 6Bis formed in the transistor, the channel width can be defined as the sumof W_(T) and the lengths of the side surfaces (W_(S1) and W_(S2)) of thesecond oxide semiconductor layer 132 in the channel width direction(i.e., W_(T)+W_(S1)+W_(S2)), and on-state current flows in thetransistor in accordance with the channel width. In the case where W_(T)is sufficiently small, current flows in the entire second oxidesemiconductor layer 132.

In other words, the transistor illustrated in FIGS. 6A and 6B has ahigher on-state current than that of the conventional transistor becausethe transistor illustrated in FIGS. 6A and 6B has an effect ofsuppressing scattering of carriers and an effect of extending theeffective channel width.

Note that in order to efficiently increase the on-state current of thetransistor when W_(S1) and W_(S2) are represented by W_(S)(W_(S1)=W_(S2)=W_(S)), a relation 0.3W_(S)≦W_(T)≦3W_(S) (W_(T) isgreater than or equal to 0.3W_(S) and less than or equal to 3W_(S)) issatisfied. Further, W_(T)/W_(S) is preferably greater than or equal to0.5 and less than or equal to 1.5, further preferably greater than orequal to 0.7 and less than or equal to 1.3. In the case whereW_(T)/W_(S)>3, the S value and the off-state current might be increased.

As described above, with the transistor of one embodiment of the presentinvention, sufficiently high on-state current can be obtained even whenthe transistor is miniaturized.

In the transistor of one embodiment of the present invention, the secondoxide semiconductor layer 132 is formed over the first oxidesemiconductor layer 131, so that an interface state is less likely to beformed. In addition, impurities do not enter the second oxidesemiconductor layer 132 from above and below because the second oxidesemiconductor layer 132 is an intermediate layer in a three-layerstructure. Since the second oxide semiconductor layer 132 is surroundedby the first oxide semiconductor layer 131 and the third oxidesemiconductor layer 133, not only can the on-state current of thetransistor be increased but also the threshold voltage can be stabilizedand the S value (subthreshold value) can be reduced. Thus, Icut (currentwhen gate voltage VG is 0 V) can be reduced and power consumption can bereduced. Further, the threshold voltage of the transistor becomesstable; thus, long-term reliability of the semiconductor device can beimproved.

The transistor of one embodiment of the present invention may include aconductive film 172 between the oxide semiconductor layer 130 and thesubstrate 110 as illustrated in FIG. 7. When the conductive film is usedas a second gate electrode, the on-state current can be furtherincreased and the threshold voltage can be controlled. In order toincrease the on-state current, for example, the gate electrode layer 170and the conductive film 172 are set to have the same potential, and thetransistor is driven as a dual-gate transistor. Further, to control thethreshold voltage, a fixed potential, which is different from apotential of the gate electrode layer 170, is supplied to the conductivefilm 172.

This embodiment can be combined as appropriate with any of the otherembodiments and an example in this specification.

Embodiment 2

In this embodiment, a method for manufacturing the transistor 100, whichis described in Embodiment 1 with reference to FIGS. 1A and 1B, isdescribed with reference to FIGS. 9A to 9C, FIGS. 10A and 10B, and FIGS.11A and 11B.

For the substrate 110, a glass substrate, a ceramic substrate, a quartzsubstrate, a sapphire substrate, or the like can be used. Alternatively,a single crystal semiconductor substrate or a polycrystallinesemiconductor substrate made of silicon, silicon carbide, or the like, acompound semiconductor substrate made of silicon germanium or the like,a silicon-on-insulator (SOI) substrate, or the like can be used. Furtheralternatively, any of these substrates further provided with asemiconductor element can be used.

The base insulating film 120 can be formed by a plasma chemical vapordeposition (CVD) method, a sputtering method, or the like using an oxideinsulating film of aluminum oxide, magnesium oxide, silicon oxide,silicon oxynitride, gallium oxide, germanium oxide, yttrium oxide,zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide,tantalum oxide, or the like; a nitride insulating film of siliconnitride, silicon nitride oxide, aluminum nitride, aluminum nitrideoxide, or the like; or a film in which any of the above materials aremixed. Alternatively, a stack including any of the above materials maybe used, and at least an upper layer of the base insulating film 120which is in contact with the oxide semiconductor layer 130 is preferablyformed using a material containing excess oxygen that might serve as asupply source of oxygen to the oxide semiconductor layer 130.

Oxygen may be added to the base insulating film 120 by an ionimplantation method, an ion doping method, a plasma immersion ionimplantation method, or the like. Adding oxygen enables the baseinsulating film 120 to supply oxygen much easily to the oxidesemiconductor layer 130.

In the case where a surface of the substrate 110 is made of an insulatorand there is no influence of impurity diffusion to the oxidesemiconductor layer 130 to be formed later, the base insulating film 120is not necessarily provided.

Next, a first oxide semiconductor film 331 to be the first oxidesemiconductor layer 131 and a second oxide semiconductor film 332 to bethe second oxide semiconductor layer 132 are deposited over the baseinsulating film 120 by a sputtering method, a CVD method, an MBE method,an atomic layer deposition (ALD) method, or a PLD method.

The first oxide semiconductor film 331 and the second oxidesemiconductor film 332 are preferably stacked successively withoutexposure to the air with the use of a multi-chamber deposition apparatus(e.g., a sputtering apparatus) including a load lock chamber. It ispreferable that each chamber of the sputtering apparatus be able to beevacuated to a high vacuum (to about 5×10⁻⁷ Pa to 1×10⁻⁴ Pa) by anadsorption vacuum pump such as a cryopump and that the chamber be ableto heat a substrate over which a film is to be deposited to 100° C. orhigher, preferably 500° C. or higher, so that water and the like actingas impurities of an oxide semiconductor are removed as much as possible.Alternatively, a combination of a turbo molecular pump and a cold trapis preferably used to prevent back-flow of a gas containing a carboncomponent, moisture, or the like from an exhaust system into thechamber.

Not only high vacuum evacuation of the chamber but also high purity of asputtering gas is necessary to obtain a highly purified intrinsic oxidesemiconductor. An oxygen gas or an argon gas used as the sputtering gasis highly purified to have a dew point of −40° C. or lower, preferably−80° C. or lower, further preferably −100° C. or lower, so that entry ofmoisture and the like into the oxide semiconductor layer can beprevented as much as possible.

For the first oxide semiconductor film 331, the second oxidesemiconductor film 332, and a third oxide semiconductor film 333 to bethe third oxide semiconductor layer 133 formed in a later step, any ofthe materials described in Embodiment 1 can be used. For example, anIn—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:3:6, 1:3:4,1:3:3, or 1:3:2 can be used for the first oxide semiconductor film 331,an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:1:1, 5:5:6,or 3:1:2 can be used for the second oxide semiconductor film 332, and anIn—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:3:6, 1:3:4,1:3:3, or 1:3:2 can be used for the third oxide semiconductor film 333.

An oxide semiconductor that can be used for each of the first oxidesemiconductor film 331, the second oxide semiconductor film 332, and thethird oxide semiconductor film 333 preferably contains at least indium(In) or zinc (Zn). Alternatively, the oxide semiconductor preferablycontains both In and Zn. In order to reduce variation in the electricalcharacteristics of the transistor including the oxide semiconductor, theoxide semiconductor preferably contains a stabilizer in addition to Inand/or Zn.

Examples of a stabilizer include gallium (Ga), tin (Sn), hafnium (Hf),aluminum (Al), and zirconium (Zr). Other examples of a stabilizerinclude lanthanoid such as lanthanum (La), cerium (Ce), praseodymium(Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd),terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm),ytterbium (Yb), and lutetium (Lu).

As the oxide semiconductor, for example, any of the following can beused: indium oxide, tin oxide, zinc oxide, an In—Zn oxide, a Sn—Znoxide, an Al—Zn oxide, a Zn—Mg oxide, a Sn—Mg oxide, an In—Mg oxide, anIn—Ga oxide, an In—Ga—Zn oxide, an In—Al—Zn oxide, an In—Sn—Zn oxide, aSn—Ga—Zn oxide, an Al—Ga—Zn oxide, a Sn—Al—Zn oxide, an In—Hf—Zn oxide,an In—La—Zn oxide, an In—Ce—Zn oxide, an In—Pr—Zn oxide, an In—Nd—Znoxide, an In—Sm—Zn oxide, an In—Eu—Zn oxide, an In—Gd—Zn oxide, anIn—Tb—Zn oxide, an In—Dy—Zn oxide, an In—Ho—Zn oxide, an In—Er—Zn oxide,an In—Tm—Zn oxide, an In—Yb—Zn oxide, an In—Lu—Zn oxide, an In—Sn—Ga—Znoxide, an In—Hf—Ga—Zn oxide, an In—Al—Ga—Zn oxide, an In—Sn—Al—Zn oxide,an In—Sn—Hf—Zn oxide, and an In—Hf—Al—Zn oxide.

Note that here, for example, an “In—Ga—Zn oxide” means an oxidecontaining In, Ga, and Zn as its main components. The In—Ga—Zn oxide maycontain a metal element other than In, Ga, and Zn. Further, in thisspecification, a film formed using an In—Ga—Zn oxide is also referred toas an IGZO film.

Alternatively, a material represented by InAlO₃(ZnO)_(m) (m>0, where mis not an integer) may be used. Note that M represents one or more metalelements selected from Ga, Y, Zr, La, Ce, and Nd. Further alternatively,a material represented by In₂SnO₅(ZnO)_(n) (n>0, where n is an integer)may be used.

Note that as described in Embodiment 1 in detail, the second oxidesemiconductor layer 132 is formed so as to have an electron affinityhigher than that of the first oxide semiconductor layer 131 and that ofthe third oxide semiconductor layer 133.

The oxide semiconductor layers are each preferably formed by asputtering method. As a sputtering method, an RF sputtering method, a DCsputtering method, an AC sputtering method, or the like can be used.

In the case of using an In—Ga—Zn oxide, a material whose atomic ratio ofIn to Ga and Zn is any of 1:1:1, 2:2:1, 2:2:3, 3:1:2, 5:5:6, 1:3:2,1:3:3, 1:3:4, 1:3:6, 1:4:3, 1:5:4, 1:6:6, 1:6:4, 1:9:6, 1:1:4, and 1:1:2can be used for the first oxide semiconductor film 331, the second oxidesemiconductor film 332, and/or the third oxide semiconductor film 333.

Note that for example, in the case where the composition of an oxidecontaining In, Ga, and Zn at the atomic ratio, In:Ga:Zn=a:b:c (a+b+c=1),is in the neighborhood of the composition of an oxide containing In, Ga,and Zn at the atomic ratio, In:Ga:Zn=A:B:C (A+B+C=1), a, b, and csatisfy the following relation: (a−A)²+(b−B)²+(c−C)²≦r², and r may be0.05, for example. The same applies to other oxides.

The indium content of the second oxide semiconductor film 332 ispreferably higher than the indium content of the first oxidesemiconductor film 331 and the indium content of the third oxidesemiconductor film 333. In an oxide semiconductor, the s orbital ofheavy metal mainly contributes to carrier transfer, and when theproportion of In in the oxide semiconductor is increased, overlap of thes orbitals is likely to be increased. Thus, an oxide having acomposition in which the proportion of In is higher than that of Ga hashigher mobility than an oxide having a composition in which theproportion of In is equal to or lower than that of Ga. For this reason,with the use of an oxide having a high indium content for the secondoxide semiconductor film 332, a transistor having high mobility can beachieved.

A structure of an oxide semiconductor film is described below.

Note that in this specification, a term “parallel” indicates that theangle formed between two straight lines is greater than or equal to −10°and less than or equal to 10°, and accordingly also includes the casewhere the angle is greater than or equal to −5° and less than or equalto 5°. In addition, a term “perpendicular” indicates that the angleformed between two straight lines is greater than or equal to 80° andless than or equal to 100°, and accordingly includes the case where theangle is greater than or equal to 85° and less than or equal to 95°.

In this specification, the trigonal and rhombohedral crystal systems areincluded in the hexagonal crystal system.

An oxide semiconductor film is classified roughly into a single-crystaloxide semiconductor film and a non-single-crystal oxide semiconductorfilm. The non-single-crystal oxide semiconductor film includes any of ac-axis aligned crystalline oxide semiconductor (CAAC-OS) film, apolycrystalline oxide semiconductor film, a microcrystalline oxidesemiconductor film, an amorphous oxide semiconductor film, and the like.

First, a CAAC-OS film is described.

The CAAC-OS film is one of oxide semiconductor films including aplurality of crystal parts, and most of the crystal parts each fitinside a cube whose one side is less than 100 nm. Thus, there is a casewhere a crystal part included in the CAAC-OS film fits inside a cubewhose one side is less than 10 nm, less than 5 nm, or less than 3 nm.

In a transmission electron microscope (TEM) image of the CAAC-OS film, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflected by a surface over which theCAAC-OS film is formed (hereinafter, a surface over which the CAAC-OSfilm is formed is referred to as a formation surface) or a top surfaceof the CAAC-OS film, and is arranged in parallel to the formationsurface or the top surface of the CAAC-OS film.

On the other hand, according to the TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface (plan TEM image), metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

From the results of the cross-sectional TEM image and the plan TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray enters a sample in a direction substantiallyperpendicular to the c-axis, a peak appears frequently when 2θ is around56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal.Here, analysis (0 scan) is performed under conditions where the sampleis rotated around a normal vector of a sample surface as an axis (φaxis) with 2θ fixed at around 56°. In the case where the sample is asingle-crystal oxide semiconductor film of InGaZnO₄, six peaks appear.The six peaks are derived from crystal planes equivalent to the (110)plane. On the other hand, in the case of a CAAC-OS film, a peak is notclearly observed even when φ scan is performed with 2θ fixed at around56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallel toa normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer arranged in a layered mannerobserved in the cross-sectional TEM image corresponds to a planeparallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axes of the crystal are aligned ina direction parallel to a normal vector of a formation surface or anormal vector of a top surface. Thus, for example, in the case where ashape of the CAAC-OS film is changed by etching or the like, the c-axismight not be necessarily parallel to a normal vector of a formationsurface or a normal vector of a top surface of the CAAC-OS film.

Further, the degree of crystallinity in the CAAC-OS film is notnecessarily uniform. For example, in the case where crystal growthleading to the CAAC-OS film occurs from the vicinity of the top surfaceof the film, the degree of the crystallinity in the vicinity of the topsurface is higher than that in the vicinity of the formation surface insome cases. Further, when an impurity is added to the CAAC-OS film, thecrystallinity in a region to which the impurity is added is changed, andthe degree of crystallinity in the CAAC-OS film varies depending onregions.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ do not appear at around36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic arrangement ofthe oxide semiconductor film by depriving the oxide semiconductor filmof oxygen and causes a reduction in crystallinity. Further, a heavymetal such as iron or nickel, argon, carbon dioxide, or the like has alarge atomic radius (or molecular radius), and thus disturbs the atomicarrangement of the oxide semiconductor film and causes a reduction incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

Further, the CAAC-OS film is an oxide semiconductor film having a lowdensity of defect states. For example, an oxygen vacancy in the oxidesemiconductor film serves as a carrier trap or a carrier generationsource in some cases when hydrogen is captured therein.

The state in which the impurity concentration is low and the density ofdefect states is low (the number of oxygen vacancies is small) isreferred to as a highly purified intrinsic state or a substantiallyhighly purified intrinsic state. A highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has fewcarrier generation sources, and thus can have a low carrier density.Thus, a transistor including the oxide semiconductor film rarely hasnegative threshold voltage (is rarely normally on). The highly purifiedintrinsic or substantially highly purified intrinsic oxide semiconductorfilm has few carrier traps. Accordingly, the transistor including theoxide semiconductor film has small variation in electricalcharacteristics and high reliability. Electric charge trapped by thecarrier traps in the oxide semiconductor film takes a long time to bereleased, and might behave like fixed electric charge. Thus, thetransistor that includes the oxide semiconductor film having highimpurity concentration and a high density of defect states has unstableelectrical characteristics in some cases.

With the use of the CAAC-OS film in a transistor, variation in theelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light is small.

Next, a microcrystalline oxide semiconductor film is described.

In an image obtained with a TEM, crystal parts cannot be found clearlyin the microcrystalline oxide semiconductor in some cases. In mostcases, the size of a crystal part in the microcrystalline oxidesemiconductor film is greater than or equal to 1 nm and less than orequal to 100 nm, or greater than or equal to 1 nm and less than or equalto 10 nm. An oxide semiconductor film including nanocrystal (nc), whichis a microcrystal with a size greater than or equal to 1 nm and lessthan or equal to 10 nm, or a size greater than or equal to 1 nm and lessthan or equal to 3 nm, is specifically referred to as a nanocrystallineoxide semiconductor (nc-OS) film. In an image of the nc-OS film obtainedwith a TEM, for example, a crystal grain cannot be observed clearly insome cases.

In the nc-OS film, a microscopic region (e.g., a region with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic order. Further, there is noregularity of crystal orientation between different crystal parts in thenc-OS film; thus, the orientation in the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor film depending on an analysis method.For example, when the nc-OS film is subjected to structural analysis byan out-of-plane method with an XRD apparatus using an X-ray having adiameter larger than that of a crystal part, a peak which shows acrystal plane does not appear. Further, a halo pattern is observed in anelectron diffraction pattern (also referred to as a selected-areaelectron diffraction pattern) of the nc-OS film obtained by using anelectron beam having a probe diameter (e.g., larger than or equal to 50nm) larger than the diameter of a crystal part. Meanwhile, spots areobserved in a nanobeam electron diffraction pattern of the nc-OS filmobtained by using an electron beam having a probe diameter (e.g., largerthan or equal to 1 nm and smaller than or equal to 30 nm) close to, orsmaller than or equal to the diameter of a crystal part. In some cases,in a nanobeam electron diffraction pattern of the nc-OS film, regionswith high luminance in a circular (ring) pattern are observed. Further,in a nanobeam electron diffraction pattern of the nc-OS film, aplurality of spots are shown in a ring-like region in some cases.

Since an nc-OS film is an oxide semiconductor film having moreregularity than an amorphous oxide semiconductor film, the nc-OS filmhas a lower density of defect states than the amorphous oxidesemiconductor film. However, there is no regularity of crystalorientation between different crystal parts in the nc-OS film; hence,the nc-OS film has a higher density of defect states than a CAAC-OSfilm.

Note that an oxide semiconductor film may be a stacked film includingtwo or more films of an amorphous oxide semiconductor film, amicrocrystalline oxide semiconductor film, and a CAAC-OS film, forexample.

A CAAC-OS film can be deposited by a sputtering method with apolycrystalline oxide semiconductor sputtering target, for example. Whenions collide with the sputtering target, a crystal region included inthe sputtering target may be separated from the target along the a-bplane; in other words, a sputtered particle having a plane parallel tothe a-b plane (a flat-plate-like sputtered particle or a pellet-likesputtered particle) might flake off from the target. In this case, theflat-plate-like or pellet-like sputtered particle is electricallycharged and thus reaches a substrate while maintaining its crystal statewithout being aggregated in plasma, whereby a CAAC-OS film can beformed.

In the case where the second oxide semiconductor film 332 is formedusing an In-M-Zn oxide (M is Ga, Y, Zr, La, Ce, or Nd) and a sputteringtarget whose atomic ratio of In to M and Zn is a₁:b₁:c₁ is used forforming the second oxide semiconductor film 332, a₁/b₁ is preferablygreater than or equal to ⅓ and less than or equal to 6, furtherpreferably greater than or equal to 1 and less than or equal to 6, andc₁/b₁ is preferably greater than or equal to ⅓ and less than or equal to6, further preferably greater than or equal to 1 and less than or equalto 6. Note that when c₁/b₁ is greater than or equal to 1 and less thanor equal to 6, a CAAC-OS film is easily formed as the second oxidesemiconductor film 332. Typical examples of the atomic ratio of In to Mand Zn of the target are 1:1:1, 3:1:2, and 5:5:6.

In the case where the first oxide semiconductor film 331 and the thirdoxide semiconductor film 333 are each formed using an In-M-Zn oxide (Mis Ga, Y, Zr, La, Ce, or Nd) and a sputtering target whose atomic ratioof In to M and Zn is a₂:b₂:c₂ is used for forming the first oxidesemiconductor film 331 and the third oxide semiconductor film 333, a₂/b₂is preferably less than a₁/b₁, and c₂/b₂ is preferably greater than orequal to ⅓ and less than or equal to 6, further preferably greater thanor equal to 1 and less than or equal to 6. Note that when c₂/b₂ isgreater than or equal to 1 and less than or equal to 6, CAAC-OS filmsare easily formed as the first oxide semiconductor film 331 and thethird oxide semiconductor film 333. Typical examples of the atomic ratioof In to M and Zn of the target are 1:3:2, 1:3:3, 1:3:4, and 1:3:6.

First heat treatment may be performed after the second oxidesemiconductor film 332 is formed. The first heat treatment may beperformed at a temperature higher than or equal to 250° C. and lowerthan or equal to 650° C., preferably higher than or equal to 300° C. andlower than or equal to 500° C., in an inert gas atmosphere, in anatmosphere containing an oxidizing gas at 10 ppm or more, or underreduced pressure. Alternatively, the first heat treatment may beperformed in such a manner that heat treatment is performed in an inertgas atmosphere, and then another heat treatment is performed in anatmosphere containing an oxidizing gas at 10 ppm or more in order tocompensate desorbed oxygen. By the first heat treatment, thecrystallinity of the second oxide semiconductor film 332 can beimproved, and in addition, impurities such as hydrogen and water can beremoved from the base insulating film 120 and the first oxidesemiconductor film 331. Note that the first heat treatment may beperformed after etching for formation of the first oxide semiconductorlayer 131 and the second oxide semiconductor layer 132 which isdescribed later.

Next, a first conductive film 340 is formed over the second oxidesemiconductor film 332. For the first conductive film 340, Al, Cr, Cu,Ta, Ti, Mo, W, or an alloy material containing any of these as its maincomponent can be used. For example, a tungsten film with a thickness of5 nm to 25 nm is formed by a sputtering method, a CVD method, or thelike.

Next, a first resist mask 400 is formed over the first conductive film340 (see FIG. 8A). It is preferable that the first resist mask 400 beformed by a photolithography process using electron beam exposure,liquid immersion exposure, or EUV exposure, for example. With such aprocess, the first resist mask 400 having an extremely minute shape canbe formed.

Next, the first conductive film 340 is selectively etched using thefirst resist mask 400 as a mask, so that a first conductive layer 341having an upper surface shape similar to an upper surface shape of thefirst resist mask 400 is formed.

Here, the first conductive layer 341 is used as a hard mask. In anetching step, the shape of a resist mask is changed because of change inquality and reduction in thickness. Thus, when the second oxidesemiconductor layer 132 and the first oxide semiconductor layer 131 areformed using only a resist mask, the second oxide semiconductor layer132 and the first oxide semiconductor layer 131 reflect the changedshape of the resist mask and thus cannot have a desired shape. When thefirst conductive layer 341 is used as a hard mask, the second oxidesemiconductor layer 132 and the first oxide semiconductor layer 131 canbe formed to have a desired shape.

The second oxide semiconductor film 332 and the first oxidesemiconductor film 331 are selectively etched, so that the second oxidesemiconductor layer 132 and the first oxide semiconductor layer 131 areformed (see FIG. 8B). Note that the base insulating film 120 may bepartly etched by over-etching the first oxide semiconductor film 331.

Next, a second resist mask is formed over the first conductive layer 341by a method similar to that of the first resist mask 400. Then, thefirst conductive layer 341 is selectively etched using the second resistmask as a mask, so that the source electrode layer 140 and the drainelectrode layer 150 are formed (see FIG. 8C). Note that the firstconductive layer 341 may be over-etched so that the second oxidesemiconductor layer 132 is partly etched.

Subsequently, the third oxide semiconductor film 333 to be the thirdoxide semiconductor layer 133 is formed over the first oxidesemiconductor layer 131, the second oxide semiconductor layer 132, thesource electrode layer 140, and the drain electrode layer 150.

Note that second heat treatment may be performed after the third oxidesemiconductor film 333 is formed. The second heat treatment can beperformed under the conditions similar to those of the first heattreatment. The second heat treatment can remove impurities such ashydrogen and water from the third oxide semiconductor film 333, thefirst oxide semiconductor layer 131, and the second oxide semiconductorlayer 132.

Next, an insulating film 360 to be the gate insulating film 160 isformed over the third oxide semiconductor film 333. The insulating film360 can be formed using aluminum oxide, magnesium oxide, silicon oxide,silicon oxynitride, silicon nitride oxide, silicon nitride, galliumoxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide,neodymium oxide, hafnium oxide, tantalum oxide, or the like. Theinsulating film 360 may be a stack including any of the above materials.The insulating film 360 can be formed by a sputtering method, a CVDmethod, an MBE method, an ALD method, a PLD method, or the like.

Then, a second conductive film 370 to be the gate electrode layer 170 isformed over the insulating film 360 (see FIG. 9A). For the secondconductive film 370, Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ru, Ag, Ta, W,or an alloy material containing any of these as its main component canbe used. The second conductive film 370 can be formed by a sputteringmethod, a CVD method, or the like. A stack including a conductive filmcontaining any of the above materials and a conductive film containingnitrogen, or a conductive film containing nitrogen may be used for thesecond conductive film 370.

After that, a third resist mask is formed over the second conductivefilm 370, and the second conductive film 370 is selectively etched usingthe third resist mask to form the gate electrode layer 170.

Then, the insulating film 360 is selectively etched using the gateelectrode layer 170 as a mask to form the gate insulating film 160.

Subsequently, the third oxide semiconductor film 333 is etched using thegate electrode layer 170 or the gate insulating film 160 as a mask toform the third oxide semiconductor layer 133 (see FIG. 9B).

The second conductive film 370, the insulating film 360, and the thirdoxide semiconductor film 333 may be etched individually or successively.Further, either dry etching or wet etching may be used as the etchingmethod, and an appropriate etching method may be selected individually.

Next, the insulating layer 180 and the insulating layer 185 are formedover the source electrode layer 140, the drain electrode layer 150, andthe gate electrode layer 170 (see FIG. 9C). The insulating layer 180 andthe insulating layer 185 can each be formed using a material and amethod which are similar to those of the base insulating film 120. Inparticular, aluminum oxide is preferably used for the insulating layer180.

Oxygen may be added to the insulating layer 180 and/or the insulatinglayer 185 by an ion implantation method, an ion doping method, a plasmaimmersion ion implantation method, or the like. Adding oxygen enablesthe insulating layer 180 and/or the insulating layer 185 to supplyoxygen much easily to the oxide semiconductor layer 130.

Next, third heat treatment may be performed. The third heat treatmentcan be performed under conditions similar to those of the first heattreatment. By the third heat treatment, excess oxygen is easily releasedfrom the base insulating film 120, the gate insulating film 160, theinsulating layer 180, and the insulating layer 185, so that oxygenvacancies in the oxide semiconductor layer 130 can be reduced.

Next, a fourth resist mask is formed over the insulating layer 185, andthe insulating layer 185, the insulating layer 180, the source electrodelayer 140, the drain electrode layer 150, the second oxide semiconductorlayer 132, and the first oxide semiconductor layer 131 are selectivelyetched using the fourth resist mask, so that the first opening 147 andthe second opening 157 are formed (see FIG. 10A). At this time, thethird opening 177 illustrated in FIG. 2A is also formed.

Note that the insulating layer 185, the insulating layer 180, the sourceelectrode layer 140, the drain electrode layer 150, the second oxidesemiconductor layer 132, and the first oxide semiconductor layer 131 maybe etched individually or successively. Further, either dry etching orwet etching may be used as the etching method, and an appropriateetching method may be selected individually.

By controlling etching conditions at this time, transistors havingdifferent structures illustrated in FIGS. 3A to 3C can be formed.

After that, the first wiring 145 and the second wiring 155 are formed tocover the first opening 147 and the second opening 157. The second oxidesemiconductor layer 132 and the source electrode layer 140 areelectrically connected to the first wiring 145, and the second oxidesemiconductor layer 132 and the drain electrode layer 150 areelectrically connected to the second wiring 155 (see FIG. 10B). Further,at this time, the third wiring 175 is formed to cover the third opening177 illustrated in FIG. 2A and is electrically connected to the gateelectrode layer 170.

Note that the first wiring 145, the second wiring 155, and the thirdwiring 175 can be formed using a material and a method similar to thoseof the source electrode layer 140, the drain electrode layer 150, or thegate electrode layer 170.

Through the above process, the transistor 100 illustrated in FIGS. 1Aand 1B can be fabricated.

A variety of films such as the metal film described in this embodimentcan be formed typically by a sputtering method or a plasma CVD method;however, these films may be formed by another method such as a thermalCVD method. A metal organic chemical vapor deposition (MOCVD) method andan ALD method are given as examples of a thermal CVD method.

A thermal CVD method has an advantage that no defect due to plasmadamage is generated since it does not utilize plasma for forming a film.

Deposition by a thermal CVD method may be performed in such a mannerthat a source gas and an oxidizer are supplied to the chamber at a time,the pressure in a chamber is set to an atmospheric pressure or a reducedpressure, and reaction is caused in the vicinity of the substrate orover the substrate.

Deposition by an ALD method may be performed in such a manner that thepressure in a chamber is set to an atmospheric pressure or a reducedpressure, source gases for reaction are sequentially introduced into thechamber, and then the sequence of the gas introduction is repeated. Forexample, two or more kinds of source gases are sequentially supplied tothe chamber by switching respective switching valves (also referred toas high-speed valves). In such a case, a first source gas is introduced,an inert gas (e.g., argon or nitrogen) or the like is introduced at thesame time as or after the introduction of the first source gas, and thena second source gas is introduced, whereby the source gases are notmixed. Note that in the case where the first source gas and the inertgas are introduced at a time, the inert gas serves as a carrier gas, andthe inert gas may also be introduced at the same time as theintroduction of the second source gas. Instead of the introduction ofthe inert gas, the first source gas may be exhausted by vacuumevacuation, and then the second source gas may be introduced. The firstsource gas is adsorbed on the surface of the substrate to form a firstlayer and then, the second source gas is introduced to react with thefirst layer; as a result, a second layer is stacked over the firstlayer, so that a thin film is formed. The sequence of the gasintroduction is repeated plural times until a desired thickness isobtained, whereby a thin film with excellent step coverage can beformed. The thickness of the thin film can be adjusted by the number ofrepetition times of the sequence of the gas introduction; therefore, anALD method makes it possible to accurately adjust a thickness and thusis suitable for manufacturing a minute FET.

In the case where a tungsten film is formed using a deposition apparatusemploying ALD, for example, a WF₆ gas and a B₂H₆ gas are sequentiallyintroduced plural times to form an initial tungsten film, and then a WF₆gas and an H₂ gas are introduced at a time, so that the tungsten film isformed. Note that an SiH₄ gas may be used instead of a B₂H₆ gas.

This embodiment can be combined as appropriate with any of the otherembodiments and an example in this specification.

Embodiment 3

In this embodiment, an example of a semiconductor device (storagedevice) which includes the transistor of one embodiment of the presentinvention, which can retain stored data even when not powered, and whichhas an unlimited number of write cycles is described with reference todrawings.

FIG. 11A is a cross-sectional view of the semiconductor device, and FIG.11B is a circuit diagram of the semiconductor device.

The semiconductor device illustrated in FIGS. 11A and 11B includes atransistor 3200 including a first semiconductor material in a lowerportion, and a transistor 3300 including a second semiconductor materialand a capacitor 3400 in an upper portion. Note that the transistor 100described in Embodiment 1 can be used as the transistor 3300.

One electrode of the capacitor 3400 is formed using the same material asa wiring layer electrically connected to a source electrode layer or adrain electrode layer of the transistor 3300, the other electrode of thecapacitor 3400 is formed using the same material as a gate electrodelayer of the transistor 3300, and a dielectric of the capacitor 3400 isformed using the same material as the insulating layer 180 and theinsulating layer 185 of the transistor 3300; thus, the capacitor 3400can be formed at the same time as the transistor 3300.

Here, the first semiconductor material and the second semiconductormaterial preferably have different energy gaps. For example, the firstsemiconductor material may be a semiconductor material (such as silicon)other than an oxide semiconductor, and the second semiconductor materialmay be the oxide semiconductor described in Embodiment 1. A transistorincluding a material other than an oxide semiconductor can operate athigh speed easily. On the other hand, a transistor including an oxidesemiconductor enables charge to be retained for a long time owing to itselectrical characteristics, that is, the low off-state current.

Although both of the above transistors are n-channel transistors in thefollowing description, it is needless to say that p-channel transistorscan be used. The specific structure of the semiconductor device, such asa material used for the semiconductor device and the structure of thesemiconductor device, needs not to be limited to that described hereexcept for the use of the transistor described in Embodiment 1, which isformed using an oxide semiconductor for retaining data.

The transistor 3200 in FIG. 11A includes a channel formation regionprovided in a substrate 3000 containing a semiconductor material (suchas crystalline silicon), impurity regions provided such that the channelformation region is provided therebetween, intermetallic compoundregions in contact with the impurity regions, a gate insulating filmprovided over the channel formation region, and a gate electrode layerprovided over the gate insulating film. Note that a transistor whosesource electrode layer and drain electrode layer are not illustrated ina drawing may also be referred to as a transistor for the sake ofconvenience. Further, in such a case, in description of a connection ofa transistor, a source region and a source electrode layer may becollectively referred to as a source electrode layer, and a drain regionand a drain electrode layer may be collectively referred to as a drainelectrode layer. That is, in this specification, the term “sourceelectrode layer” might include a source region.

An element isolation insulating layer 3100 is formed on the substrate3000 so as to surround the transistor 3200, and an insulating layer 3150is formed so as to cover the transistor 3200. Note that the elementisolation insulating layer 3100 can be formed by an element isolationtechnique such as local oxidation of silicon (LOCOS) or shallow trenchisolation (STI).

In the case where the transistor 3200 is formed using a crystallinesilicon substrate, for example, the transistor 3200 can operate at highspeed. Thus, when the transistor is used as a reading transistor, datacan be read at high speed.

The transistor 3300 is provided over the insulating layer 3150, and thewiring layer electrically connected to the source electrode layer or thedrain electrode layer of the transistor 3300 serves as the one electrodeof the capacitor 3400. Further, the one electrode of the capacitor 3400is electrically connected to the gate electrode layer of the transistor3200.

The transistor 3300 in FIG. 11A is a top-gate transistor in which achannel is formed in an oxide semiconductor layer. Since the off-statecurrent of the transistor 3300 is low, stored data can be retained for along period owing to such a transistor. In other words, refreshoperation becomes unnecessary or the frequency of the refresh operationin a semiconductor storage device can be extremely low, which leads to asufficient reduction in power consumption.

Further, an electrode 3250 is provided so as to overlap with thetransistor 3300 with the insulating layer 3150 provided therebetween. Bysupplying an appropriate potential to the electrode 3250 to be used as asecond gate electrode, the threshold voltage of the transistor 3300 canbe controlled. In addition, long-term reliability of the transistor 3300can be improved. When the electrode operates with the same potential asthat of the gate electrode of the transistor 3300, on-state current canbe increased. Note that the electrode 3250 is not necessarily provided.

The transistor 3300 and the capacitor 3400 can be formed over thesubstrate over which the transistor 3200 is formed as illustrated inFIG. 11A, which enables the degree of the integration of thesemiconductor device to be increased.

An example of a circuit configuration of the semiconductor device inFIG. 11A is illustrated in FIG. 11B.

In FIG. 11B, a first wiring 3001 is electrically connected to a sourceelectrode layer of the transistor 3200. A second wiring 3002 iselectrically connected to a drain electrode layer of the transistor3200. A third wiring 3003 is electrically connected to one of the sourceelectrode layer and the drain electrode layer of the transistor 3300. Afourth wiring 3004 is electrically connected to the gate electrode layerof the transistor 3300. The gate electrode layer of the transistor 3200and the other of the source electrode layer and the drain electrodelayer of the transistor 3300 are electrically connected to the oneelectrode of the capacitor 3400. A fifth wiring 3005 is electricallyconnected to the other electrode of the capacitor 3400. Note that acomponent corresponding to the electrode 3250 is not illustrated.

The semiconductor device in FIG. 11B utilizes a feature that thepotential of the gate electrode layer of the transistor 3200 can beretained, and thus enables writing, retaining, and reading of data asfollows.

Writing and retaining of data are described. First, the potential of thefourth wiring 3004 is set to a potential at which the transistor 3300 isturned on, so that the transistor 3300 is turned on. Accordingly, thepotential of the third wiring 3003 is supplied to the gate electrodelayer of the transistor 3200 and the capacitor 3400. That is, apredetermined charge is supplied to the gate electrode layer of thetransistor 3200 (writing). Here, one of two kinds of charges providingdifferent potential levels (hereinafter referred to as a low-levelcharge and a high-level charge) is supplied. After that, the potentialof the fourth wiring 3004 is set to a potential at which the transistor3300 is turned off, so that the transistor 3300 is turned off. Thus, thecharge supplied to the gate electrode layer of the transistor 3200 isretained (retaining).

Since the off-state current of the transistor 3300 is extremely low, thecharge of the gate electrode layer of the transistor 3200 is retainedfor a long time.

Next, reading of data is described. An appropriate potential (a readingpotential) is supplied to the fifth wiring 3005 while a predeterminedpotential (a constant potential) is supplied to the first wiring 3001,whereby the potential of the second wiring 3002 varies depending on theamount of charge retained in the gate electrode layer of the transistor3200. This is because in general, in the case of using an n-channeltransistor as the transistor 3200, an apparent threshold voltage V_(th)_(_) _(H) at the time when the high-level charge is given to the gateelectrode layer of the transistor 3200 is lower than an apparentthreshold voltage V_(th) _(_) _(L) at the time when the low-level chargeis given to the gate electrode layer of the transistor 3200. Here, anapparent threshold voltage refers to the potential of the fifth wiring3005 which is needed to turn on the transistor 3200. Thus, the potentialof the fifth wiring 3005 is set to a potential V₀ which is betweenV_(th) _(_) _(H) and V_(th) _(_) _(L), whereby charge supplied to thegate electrode layer of the transistor 3200 can be determined. Forexample, in the case where the high-level charge is supplied in writingand the potential of the fifth wiring 3005 is V₀ (>V_(th) _(_) _(H)),the transistor 3200 is turned on. In the case where the low-level chargeis supplied in writing, even when the potential of the fifth wiring 3005is V₀ (V_(th) _(_) _(L)), the transistor 3200 remains off. Thus, thedata retained in the gate electrode layer can be read by determining thepotential of the second wiring 3002.

Note that in the case where memory cells are arrayed, it is necessarythat only data of a desired memory cell be able to be read. The fifthwiring 3005 in the case where data is not read may be supplied with apotential at which the transistor 3200 is turned off regardless of thestate of the gate electrode layer, that is, a potential lower thanV_(th) _(_) _(H). Alternatively, the fifth wiring 3005 may be suppliedwith a potential at which the transistor 3200 is turned on regardless ofthe state of the gate electrode layer, that is, a potential higher thanV_(th) _(_) _(L).

When including a transistor having a channel formation region formedusing an oxide semiconductor and having an extremely low off-statecurrent, the semiconductor device described in this embodiment canretain stored data for an extremely long time. In other words, refreshoperation becomes unnecessary or the frequency of the refresh operationcan be extremely low, which leads to a sufficient reduction in powerconsumption. Moreover, stored data can be retained for a long time evenwhen power is not supplied (note that a potential is preferably fixed).

Further, in the semiconductor device described in this embodiment, highvoltage is not needed for writing data and there is no problem ofdeterioration of elements. Unlike in a conventional nonvolatile memory,for example, it is not necessary to inject and extract electrons intoand from a floating gate; thus, a problem such as deterioration of agate insulating film is unlikely to be caused. That is, thesemiconductor device of the disclosed invention does not have a limit onthe number of times data can be rewritten, which is a problem of aconventional nonvolatile memory, and the reliability thereof isdrastically improved. Furthermore, data is written depending on thestate of the transistor (on or off), whereby high-speed operation can beeasily achieved.

As described above, a miniaturized and highly-integrated semiconductordevice having high electrical characteristics can be provided.

This embodiment can be combined as appropriate with any of the otherembodiments and an example in this specification.

Embodiment 4

In this embodiment, a semiconductor device including the transistor ofone embodiment of the present invention, which can retain stored dataeven when not powered, which does not have a limit on the number ofwrite cycles, and which has a structure different from that described inEmbodiment 3, is described.

FIG. 12 illustrates an example of a circuit configuration of thesemiconductor device. In the semiconductor device, a first wiring 4500is electrically connected to a source electrode layer of a transistor4300, a second wiring 4600 is electrically connected to a gate electrodelayer of the transistor 4300, and a drain electrode layer of thetransistor 4300 is electrically connected to a first terminal of acapacitor 4400. Note that the transistor 100 described in Embodiment 1can be used as the transistor 4300 included in the semiconductor device.The first wiring 4500 can serve as a bit line and the second wiring 4600can serve as a word line.

The semiconductor device (a memory cell 4250) can have a connection modesimilar to that of the transistor 3300 and the capacitor 3400illustrated in FIGS. 11A and 11B. Thus, the capacitor 4400 can be formedthrough the same process and at the same time as the transistor 4300 ina manner similar to that of the capacitor 3400 described in Embodiment3.

Next, writing and retaining of data in the semiconductor device (thememory cell 4250) illustrated in FIG. 12 are described.

First, a potential at which the transistor 4300 is turned on is suppliedto the second wiring 4600, so that the transistor 4300 is turned on.Accordingly, the potential of the first wiring 4500 is supplied to thefirst terminal of the capacitor 4400 (writing). After that, thepotential of the second wiring 4600 is set to a potential at which thetransistor 4300 is turned off, so that the transistor 4300 is turned offThus, the potential of the first terminal of the capacitor 4400 isretained (retaining).

The transistor 4300 including an oxide semiconductor has an extremelylow off-state current. For that reason, the potential of the firstterminal of the capacitor 4400 (or a charge accumulated in the capacitor4400) can be retained for an extremely long time by turning off thetransistor 4300.

Next, reading of data is described. When the transistor 4300 is turnedon, the first wiring 4500 which is in a floating state and the capacitor4400 are electrically connected to each other, and the charge isredistributed between the first wiring 4500 and the capacitor 4400. As aresult, the potential of the first wiring 4500 is changed. The amount ofchange in potential of the first wiring 4500 varies depending on thepotential of the first terminal of the capacitor 4400 (or the chargeaccumulated in the capacitor 4400).

For example, the potential of the first wiring 4500 after the chargeredistribution is (C_(B)×V_(B0)+C×V)/(C_(B)+C), where V is the potentialof the first terminal of the capacitor 4400, C is the capacitance of thecapacitor 4400, C_(B) is the capacitance component of the first wiring4500, and V_(B0) is the potential of the first wiring 4500 before thecharge redistribution. Thus, it can be found that, assuming that thememory cell 4250 is in either of two states in which the potential ofthe first terminal of the capacitor 4400 is V₁ and V₀ (V₁>V₀), thepotential of the first wiring 4500 in the case of retaining thepotential V₁ (=(C_(B)×V_(B0)+C×V₁)/(C_(B)+C)) is higher than thepotential of the first wiring 4500 in the case of retaining thepotential V₀ (=(C_(B)×V_(B0)+C×V₀)/(C_(B)+C)).

Then, by comparing the potential of the first wiring 4500 with apredetermined potential, data can be read.

As described above, the semiconductor device (the memory cell 4250)illustrated in FIG. 12 can retain charge that is accumulated in thecapacitor 4400 for a long time because the off-state current of thetransistor 4300 is extremely low. In other words, refresh operationbecomes unnecessary or the frequency of the refresh operation can beextremely low, which leads to a sufficient reduction in powerconsumption. Moreover, stored data can be retained for a long time evenwhen power is not supplied.

A substrate over which a driver circuit for the memory cell 4250 isformed and the memory cell 4250 illustrated in FIG. 12 are preferablystacked. When the memory cell 4250 and the driver circuit are stacked,the size of the semiconductor device can be reduced. Note that there isno limitation on the numbers of the memory cells 4250 and the drivercircuits which are stacked.

It is preferable that a semiconductor material of a transistor includedin the driver circuit be different from that of the transistor 4300. Forexample, silicon, germanium, silicon germanium, silicon carbide, orgallium arsenide can be used, and a single crystal semiconductor ispreferably used. A transistor formed using such a semiconductor materialcan operate at higher speed than a transistor formed using an oxidesemiconductor and is suitable for the driver circuit for the memory cell4250.

As described above, a miniaturized and highly-integrated semiconductordevice having high electrical characteristics can be provided.

This embodiment can be combined as appropriate with any of the otherembodiments and an example in this specification.

Embodiment 5

The transistor described in Embodiment 1 can be used in a semiconductordevice such as a display device, a storage device, a CPU, a digitalsignal processor (DSP), an LSI such as a custom LSI or a programmablelogic device (PLD), or a radio frequency identification (RF-ID). In thisembodiment, electronic devices each including the semiconductor devicewill be described.

Examples of the electronic devices having the semiconductor devicesinclude display devices of televisions, monitors, and the like, lightingdevices, personal computers, word processors, image reproductiondevices, portable audio players, radios, tape recorders, stereos,phones, cordless phones, mobile phones, car phones, transceivers,wireless devices, game machines, calculators, portable informationterminals, electronic notebooks, e-book readers, electronic translators,audio input devices, video cameras, digital still cameras, electricshavers, IC chips, high-frequency heating appliances such as microwaveovens, electric rice cookers, electric washing machines, electric vacuumcleaners, air-conditioning systems such as air conditioners,dishwashers, dish dryers, clothes dryers, futon dryers, electricrefrigerators, electric freezers, electric refrigerator-freezers,freezers for preserving DNA, radiation counters, and medical equipmentsuch as dialyzers and X-ray diagnostic equipment. In addition, theexamples of the electronic devices include alarm devices such as smokedetectors, heat detectors, gas alarm devices, and security alarmdevices. Further, the examples of the electronic devices also includeindustrial equipment such as guide lights, traffic lights, beltconveyors, elevators, escalators, industrial robots, and power storagesystems. In addition, moving objects and the like driven by fuel enginesand electric motors using power from non-aqueous secondary batteries arealso included in the category of electronic devices. Examples of themoving objects include electric vehicles (EV), hybrid electric vehicles(HEV) which include both an internal-combustion engine and a motor,plug-in hybrid electric vehicles (PHEV), tracked vehicles in whichcaterpillar tracks are substituted for wheels of these vehicles,motorized bicycles including motor-assisted bicycles, motorcycles,electric wheelchairs, golf carts, boats or ships, submarines,helicopters, aircrafts, rockets, artificial satellites, space probes,planetary probes, and spacecrafts. Some specific examples of theseelectronic devices are illustrated in FIGS. 13A to 13C.

In a television set 8000 illustrated in FIG. 13A, a display portion 8002is incorporated in a housing 8001. The display portion 8002 can displayan image and a speaker portion 8003 can output sound. A storage deviceincluding the transistor of one embodiment of the present invention canbe used for a driver circuit for operating the display portion 8002.

The television set 8000 may also include a CPU 8004 for performinginformation communication or a memory. For the CPU 8004 and the memory,a CPU or a storage device including the transistor of one embodiment ofthe present invention can be used.

An alarm device 8100 illustrated in FIG. 13A is a residential firealarm, which is an example of an electronic device including a sensorportion 8102 for smoke or heat and a microcomputer 8101. Note that themicrocomputer 8101 includes a storage device or a CPU including thetransistor of one embodiment of the present invention.

An air conditioner which includes an indoor unit 8200 and an outdoorunit 8204 illustrated in FIG. 13A is an example of an electronic deviceincluding the transistor, the storage device, the CPU, or the likedescribed in any of the above embodiments. Specifically, the indoor unit8200 includes a housing 8201, an air outlet 8202, a CPU 8203, and thelike. Although the CPU 8203 is provided in the indoor unit 8200 in FIG.13A, the CPU 8203 may be provided in the outdoor unit 8204.Alternatively, the CPU 8203 may be provided in both the indoor unit 8200and the outdoor unit 8204. By using any of the transistors of oneembodiment of the present invention for the CPU in the air conditioner,a reduction in power consumption of the air conditioner can be achieved.

An electric refrigerator-freezer 8300 illustrated in FIG. 13A is anexample of an electronic device including the transistor, the storagedevice, the CPU, or the like described in any of the above embodiments.Specifically, the electric refrigerator-freezer 8300 includes a housing8301, a door for a refrigerator 8302, a door for a freezer 8303, a CPU8304, and the like. In FIG. 13A, the CPU 8304 is provided in the housing8301. When the transistor of one embodiment of the present invention isused for the CPU 8304 of the electric refrigerator-freezer 8300, areduction in power consumption of the electric refrigerator-freezer 8300can be achieved.

FIGS. 13B and 13C illustrate an example of an electric vehicle which isan example of an electronic device. An electric vehicle 9700 is equippedwith a secondary battery 9701. The output of the electric power of thesecondary battery 9701 is adjusted by a circuit 9702 and the electricpower is supplied to a driving device 9703. The circuit 9702 iscontrolled by a processing unit 9704 including a ROM, a RAM, a CPU, orthe like which is not illustrated. When the transistor of one embodimentof the present invention is used for the CPU in the electric vehicle9700, a reduction in power consumption of the electric vehicle 9700 canbe achieved.

The driving device 9703 includes a DC motor or an AC motor either aloneor in combination with an internal-combustion engine. The processingunit 9704 outputs a control signal to the circuit 9702 on the basis ofinput data such as data of operation (e.g., acceleration, deceleration,or stop) by a driver or data during driving (e.g., data on an upgrade ora downgrade, or data on a load on a driving wheel) of the electricvehicle 9700. The circuit 9702 adjusts the electric energy supplied fromthe secondary battery 9701 in accordance with the control signal of theprocessing unit 9704 to control the output of the driving device 9703.In the case where the AC motor is mounted, although not illustrated, aninverter which converts a direct current into an alternate current isalso incorporated.

This embodiment can be combined as appropriate with any of the otherembodiments and an example in this specification.

EXAMPLE

In this example, the electrical characteristics of a transistor of oneembodiment of the present invention are described.

First, a method for manufacturing a transistor is described. Thetransistor in this example has the structure illustrated in FIGS. 15Aand 15B.

As a substrate, a glass substrate was used, and a silicon oxynitridefilm was formed over the glass substrate by a plasma CVD method.

Next, a first oxide semiconductor film with a thickness of approximately10 nm and a second oxide semiconductor film with a thickness ofapproximately 40 nm were formed in this order over the siliconoxynitride film by a sputtering method. Note that an IGZO film having acomposition ratio of In:Ga:Zn=1:3:2 and an IGZO film having acomposition ratio of In:Ga:Zn=1:1:1 or In:Ga:Zn=3:1:2 were used as thefirst oxide semiconductor film and the second oxide semiconductor film,respectively.

Next, a 15-nm-thick tungsten film and an organic resin were formed overthe second oxide semiconductor film, a negative resist film was formed,exposure was performed on the resist film by scanning with an electronbeam, and development treatment was performed, so that a first resistmask was formed.

Then, by using the first resist mask, the organic resin and the tungstenfilm were selectively etched. A dry etching apparatus using inductivelycoupled plasma (ICP) was used for the etching.

Next, the first resist mask and the organic resin were removed byashing. Then, the first oxide semiconductor film and the second oxidesemiconductor film were selectively etched using the tungsten film as amask, so that a stack of a first oxide semiconductor layer, a secondoxide semiconductor layer, and the tungsten film was formed.

Next, a second resist mask was formed over the tungsten film, and thetungsten film was selectively etched using the second resist mask, sothat a source electrode layer and a drain electrode layer were formed.

Next, a 5-nm-thick third oxide semiconductor film was formed over theoxide semiconductor layers, the source electrode layer, and the drainelectrode layer by a sputtering method. Note that an IGZO film having acomposition ratio of In:Ga:Zn 1:3:2 was used as the third oxidesemiconductor film.

Next, a 10-nm-thick silicon oxynitride film to be a gate insulating filmwas formed over the third oxide semiconductor film by a plasma CVDmethod.

Next, a 10-nm-thick titanium nitride film and a 10-nm-thick tungstenfilm were successively formed by a sputtering method. After that, athird resist mask was formed over the tungsten film.

Next, the titanium nitride film and the tungsten film were selectivelyetched using the third resist mask, so that a gate electrode layer wasformed.

Next, a fourth resist mask was formed over the gate electrode layer andthe gate insulating film, and the gate insulating film and the thirdoxide semiconductor film were selectively etched using the fourth resistmask, so that the gate insulating film and a third oxide semiconductorlayer having shapes illustrated in FIGS. 15A and 15B were formed.

Next, an aluminum oxide film and a silicon oxynitride film were formedas insulating layers.

Through the above process, the transistor of one embodiment of thepresent invention (corresponding to the model (b) illustrated in FIG.16B) was fabricated.

Further, a transistor having a conventional structure (corresponding tothe model (a) illustrated in FIG. 16A) was also fabricated by changingpart of the above process.

Next, electrical characteristics of the fabricated transistors aredescribed.

FIG. 17A shows Id-Vg characteristics of the transistor having theconventional structure. The composition ratio of the second oxidesemiconductor layer of the transistor was In:Ga:Zn=1:1:1. Thefield-effect mobility of the transistor was approximately 14 cm²/Vs andthe S value thereof was approximately 105 mV/decade; thus, favorablecharacteristics were obtained.

FIG. 17B shows Id-Vg characteristics of the transistor of one embodimentof the present invention. The composition ratio of the second oxidesemiconductor layer of the transistor was In:Ga:Zn=3:1:2. Thefield-effect mobility of the transistor was approximately 21 cm²/Vs andthe S value thereof was approximately 90 mV/decade; thus, the obtainedcharacteristics were more favorable than those of the transistor havingthe conventional structure.

Here, in the case where the composition ratio of the second oxidesemiconductor layer used in the transistor having the conventionalstructure was In:Ga:Zn=3:1:2, a field-effect mobility of approximately100 cm²Ns was obtained; however, favorable characteristics were notobtained (e.g., the threshold voltage was largely shifted in thenegative direction). Further, in the case where the composition ratio ofthe second oxide semiconductor layer used in the transistor of oneembodiment of the present invention was In:Ga:Zn=1:1:1, an on-statecurrent and a field-effect mobility were lower than those in Id-Vgcharacteristics in FIG. 17A.

In other words, it is found that when an appropriate material isselected for the oxide semiconductor layer, the transistor of oneembodiment of the present invention can have more favorable electricalcharacteristics than the transistor having the conventional structure.

Note that this example can be combined with any of the embodiments inthis specification as appropriate.

EXPLANATION OF REFERENCE

-   100: transistor, 110: substrate, 120: base insulating film, 130:    oxide semiconductor layer, 131: first oxide semiconductor layer,    132: second oxide semiconductor layer, 133: third oxide    semiconductor layer, 135: boundary, 137: channel region, 140: source    electrode layer, 145: wiring, 147: first opening, 150: drain    electrode layer, 155: wiring, 157: second opening, 160: gate    insulating film, 170: gate electrode layer, 172: conductive film,    175: wiring, 177: third opening, 180: insulating layer, 185:    insulating layer, 331: first oxide semiconductor film, 332: second    oxide semiconductor film, 333: third oxide semiconductor film, 340:    first conductive film, 341: first conductive layer, 360: insulating    film, 370: second conductive film, 400: resist mask, 3000:    substrate, 3001: wiring, 3002: wiring, 3003: wiring, 3004: wiring,    3005: wiring, 3100: element isolation insulating layer, 3150:    insulating layer, 3200: transistor, 3250: electrode, 3300:    transistor, 3400: capacitor, 4250: memory cell, 4300: transistor,    4400: capacitor, 4500: wiring, 4600: wiring, 8000: television set,    8001: housing, 8002: display portion, 8003: speaker portion, 8004:    CPU, 8100: alarm device, 8101: microcomputer, 8102: sensor portion,    8200: indoor unit, 8201: housing, 8202: air outlet, 8203: CPU, 8204:    outdoor unit, 8300: electric refrigerator-freezer, 8301: housing,    8302: door for refrigerator, 8303: door for freezer, 8304: CPU,    9700: electric vehicle, 9701: secondary battery, 9702: circuit,    9703: driving device, and 9704: processing unit.

This application is based on Japanese Patent Application serial no.2013-099534 filed with Japan Patent Office on May 9, 2013, the entirecontents of which are hereby incorporated by reference.

1. A semiconductor device comprising: a first oxide semiconductor layer;a source electrode layer over the first oxide semiconductor layer, thesource electrode layer including: a first tapered side surface; and asecond side surface aligned with a first edge of the first oxidesemiconductor layer; a drain electrode layer over the first oxidesemiconductor layer, the drain electrode layer including: a thirdtapered side surface; and a fourth side surface aligned with a secondedge of the first oxide semiconductor layer; a second oxidesemiconductor layer over the first oxide semiconductor layer; a thirdoxide semiconductor layer under the first oxide semiconductor layer; afirst insulating layer over the second oxide semiconductor layer; a gateelectrode layer over the first insulating layer, the gate electrodelayer overlapping the first tapered side surface and the third taperedside surface; and a second insulating layer over the gate electrodelayer, the second insulating layer including oxygen and aluminum,wherein the source electrode layer is in direct contact with a firstpart of a top surface of the first oxide semiconductor layer, whereinthe source electrode layer is not in direct contact with any sidesurface of the first oxide semiconductor layer, wherein the drainelectrode layer is in direct contact with a second part of the topsurface of the first oxide semiconductor layer, wherein the drainelectrode layer is not in direct contact with any side surface of thefirst oxide semiconductor layer, wherein each of the source electrodelayer and the drain electrode layer includes a metal nitride, whereinthe second oxide semiconductor layer is in direct contact with a part ofa top surface of the source electrode layer and a part of a top surfaceof the drain electrode layer, wherein energy of a conduction bandminimum of the second oxide semiconductor layer is closer to a vacuumlevel than energy of a conduction band minimum of the first oxidesemiconductor layer by greater than or equal to 0.05 eV and less than orequal to 2 eV, and wherein energy of a conduction band minimum of thethird oxide semiconductor layer is closer to a vacuum level than theenergy of the conduction band minimum of the first oxide semiconductorlayer by greater than or equal to 0.05 eV and less than or equal to 2eV.
 2. The semiconductor device according to claim 1, wherein the firstoxide semiconductor layer, the second oxide semiconductor layer, and thethird oxide semiconductor layer are each an In-M-Zn oxide, wherein M isone of Al, Ti, Ga, Y, Zr, La, Ce, Nd, and Hf, and wherein an atomicratio of M to In in each of the second oxide semiconductor layer and thethird oxide semiconductor layer is higher than an atomic ratio of M toIn in the first oxide semiconductor layer.
 3. The semiconductor deviceaccording to claim 1, wherein the first oxide semiconductor layer, thesecond oxide semiconductor layer, and the third oxide semiconductorlayer each include crystals in which c-axes are aligned.
 4. Thesemiconductor device according to claim 1, wherein the metal nitride isone of tantalum nitride and titanium nitride.
 5. An electronic devicecomprising the semiconductor device according to claim
 1. 6. Asemiconductor device comprising: a first oxide semiconductor layer; asource electrode layer over the first oxide semiconductor layer, thesource electrode layer including: a first tapered side surface; and asecond side surface aligned with a first edge of the first oxidesemiconductor layer; a drain electrode layer over the first oxidesemiconductor layer, the drain electrode layer including: a thirdtapered side surface; and a fourth side surface aligned with a secondedge of the first oxide semiconductor layer; a second oxidesemiconductor layer over the first oxide semiconductor layer; and athird oxide semiconductor layer under the first oxide semiconductorlayer; a first insulating layer over the second oxide semiconductorlayer; a gate electrode layer over the first insulating layer, the gateelectrode layer overlapping the first tapered side surface and the thirdtapered side surface; and a second insulating layer over the gateelectrode layer, the second insulating layer including oxygen andaluminum, wherein the source electrode layer is in direct contact with afirst part of a top surface of the first oxide semiconductor layer,wherein the source electrode layer is not in direct contact with anyside surface of the first oxide semiconductor layer, wherein the drainelectrode layer is in direct contact with a second part of the topsurface of the first oxide semiconductor layer, wherein the drainelectrode layer is not in direct contact with any side surface of thefirst oxide semiconductor layer, wherein each of the source electrodelayer and the drain electrode layer includes a metal nitride, whereinthe second oxide semiconductor layer is in direct contact with a part ofa top surface of the source electrode layer and a part of a top surfaceof the drain electrode layer, wherein the first oxide semiconductorlayer, the second oxide semiconductor layer, and the third oxidesemiconductor layer are each an In-M-Zn oxide, wherein M is one of Al,Ti, Ga, Y, Zr, La, Ce, Nd, and Hf, and wherein an atomic ratio of M toIn in each of the second oxide semiconductor layer and the third oxidesemiconductor layer is higher than an atomic ratio of M to In in thefirst oxide semiconductor layer.
 7. The semiconductor device accordingto claim 6, wherein the first oxide semiconductor layer, the secondoxide semiconductor layer, and the third oxide semiconductor layer eachinclude crystals in which c-axes are aligned.
 8. The semiconductordevice according to claim 6, wherein the metal nitride is one oftantalum nitride and titanium nitride.
 9. An electronic devicecomprising the semiconductor device according to claim 6.